We stayed at the Signature from June 30th through July 5th 2007 and generally had a very nice vacation. We booked two rooms (studio suites), one for my 86 year old father-in-law and one for my wife and myself (early 50's) and our 9 year old daughter.
First the Good:
The Signature is a modern, nicely designed hotel with three towers - comparable in appointments to a Hyatt. We stayed in Tower 1 which had a several advantages: we were closer to the main MGM Grand and its facilities; it had a Starbucks and a nice quiet lounge/bar in the Lobby and it had a nice little gym of its own. The rooms in the Signature are very large and nicely equipped by any standards but particularly in Las Vegas where hotels often make rooms not particularly comfortable so you will spend more time in their Casino. It had a nice sized sitting area with convertible sofa which our daughter used, a very nice kitchenette area with small refrigerator, sink, two burner stove, microwave, toaster, blender and plates/glasses and silverware for two; a really large bathroom with oversized spa tub and separate glass-walled shower, two sinks and separate toilet.
The best feature of the Signature is that it is Non-Smoking everywhere: rooms, lounges, coffee shop, etc. It is a much needed break from the blue haze of the MGM Grand (more about this later). Each tower also has a separate pool and sitting area if you want to take a break from the crowds of the MGM main pool area and just sit quietly by a pool and read or snooze.
The best lunch place we found at the MGM Grand was "Wichcraft," a panini sandwhich place in the studio walk area connecting the casino and pool complex. There is also a food court of sorts for burgers and hot dogs pizza and ice-cream.
We checked out the reviews here before we booked and noted the complaints about the long walk from towers to MGM Grand main complex. They are partly right. With my 86 year old father-in-law with us, distance was always an issue. It wasn't a problem getting from Tower 1 to the MGM Grand - this has movable walkways, is completely enclosed, air-conditioned and smoke free. The walkway exits into the MGM studio walkway near the entrance to the pool complex, spa and main gym - if these are your destinations, the distance is not a problem; it's a five minute walk/ride. It's getting to the casino and the main restaurants and entertainment that is the problem - a good twenty minute walk and much of it through a heavy blue haze of cigarette smoke (more about this later).
I got a massage at the spa (a birthday present from my wife) and enjoyed it. It was over-priced as almost all hotel spa services are but at least it was competent and professionally done. I also used the gym (complimentary with spa services) and it had nice equipment. The gym at the tower is smaller but adequately equipped and it's included in the resort fee - the main gym costs $25 to use without spa services.
We saw two great shows: The Beatles Love Circque du Soleil at the MIrage and Celine Dion's concert at Caesar's. Both were fabulous and worth the money (although Celine's is especially pricey, even by Las Vegas standards). If you can only see one, take in the Beatles Love show - it is unforgettable.
The Bad:
Top of the list is the sheer heat of Las Vegas in the summer time. We were there during the hottest week on record, temperatures were routinely above 110 and hit 116 on our last day there. It made walking outside of air-conditioned surroundings virtually impossible, particularly for my father-in-law and daughter. Although it was especially hot that week, normal weeks during the summer in Las Vegas are in the 106 - 108 range. If you are sensitive to heat or like to spend time walking outdoors, Las Vegas is not the place for you in the summer time.
Although there are nice kitchen facilities in the Signature rooms, there is no place to get basic groceries like milk, bread, eggs, etc. for breakfast. I had to walk a mile or so through the entire casino and out into the steamy strip to find a drugstore to buy milk for breakfast. If you intend to use those nice kitchen facilities, bring your own stuff. You won't find it anywhere in that huge hotel complex.
Although we generally found the staff at the Signature both pleasant and helpful, the concierages we consulted for restaurant ideas and transportation info were consistently off the mark. They were good at arranging things if you knew what you wanted but were clueless when it came to suggesting things. There are gudebooks in the hotel, in taxi's and on the street. Use them - they have good info on shows, restaurants and transportation and discount coupons as well.
Most of the restaurants are in, around or near the casino which makes them both distant and very smoky. MGM Grand definetly needs to upgrade their ventillation system. Even though the restaurants are non-smoking, they all smell like the bottom of an ashtray, not very appetizing.
The Ugly:
It had to be the ever present smoke. Walking through the casino was like walking through bus exhaust - a thick fog of smoke. My daughter refused to walk through the casino a second time after her first teary, coughing experience. My wife's eyes reacted to the smoke by swelling up and staying that way for five days, until we left. Personally, I am not that sensitive to cigarette smoke but was choked by it as well. It was like getting a whiff of tear gas - irritating and tearing the eyes, burning and closing the throat - not pleasant. It very much limited what we could do and where we could go in the hotel and with the summer heat making outdoor walking impossible, didn't leave a lot of good options. We went off site by taxi for most meals and all shows.
Bottom Line:
Very nice hotel, especially for Las Vegas with modest cost and very nice appointments. If you are sensitive to heat - don't go to Las Vegas in the summer time. If you are senstive to smoke, don't go to this hotel unless you intend to stay clear of the MGM Grand and just stay in the Signature.

Chic spot, refreshing and elegant diversion from the Strip madness (but only 8 min walk away).

Just Starbucks for vittles, but we hosted a catered bruch from room service that was perfectly prepared, served and well-priced.

If after table burns, you want a diversion in total elegance, you'd like this place. (We have 40 years of visits 2-3x's per year)

Just returned from a four day stay in Vegas with wife and 15 & 19 year old kids. The good news. Best appointed rooms thus far in Vegas. Elegant, yet understated....unlike the rest of Vegas. Stayed in a high floor 1 bdrm suite and adjacent king room. Facility was very nice but the service needs to improve. Arrived at the hotel with family and numerous baggage in tow, had to request help from bellman who just looked at us upon arrival. Maid service was abysmal. Last evening at the hotel made 3 phone calls to request the initial cleaning of one of our rooms during the late afternoon. Arrived back at the hotel at 10:30 pm after a show to find a maid finally in our room with not much happening. Next day at check out, asked to speak to the hotel manager, who told the on duty manager to see me, who told the room service manager to see me, who told a supervisor to see me, who told another non-English speaking person to talk to me and I unfortunately did not speak in that person's native tongue...you get the idea. Great facility but lacking in service.

After reading the reviews on Tripadvisor I decided to check out the Signature at MGM. I was attending a conference at the Riviera but I KNEW I didn't want to stay there. The MGM has the monorail attached to it and is a big enough complex that I would never need to go outdoors if I didn't want to.

The cab ride in was cool..the gated entrance feels somewhat exclusive..they have to check your name on a list. Upon arriving in the lobby at Tower 1 (there are three towers), it was a very different feel from previous LV hotel stays where there are TONS of guests. This is much more intimate and a more relaxing feel. As others have written, if you want ultra excitement, this hotel might not be for you.

My room had a view of the strip, flat screen TV was great with a crisp picture and free HBO, etc. My only beef was no mini-bar in the rooms. I also had no idea where to buy supplies for the mini kitchenette that was in my room. Bed was very comfortable, bathroom was big and pristine. Chairs at the desk were not the most comfortable if you're doing work on a laptop, but it's not too big a deal.

Walk to the poker room was timed at about 8-9 minutes. That is on the
far side of the MGM casino and I didn't think it was too bad a walk. My walk to the monorail in the mornings was more like 3-4 minutes.

There was never a wait for a taxi, the exercise room was fantastic and not busy, and overall I would recommend this hotel to others, provided:
a) you're OK with the 5-10 minute walk from the main MGM complex
b) you don't want a hotel with ultra excitement - the Signature is for a more relaxing experience.

Only other complaint is I found the Starbucks expensive.

All in all a great experience and I would stay again. Compared to my stay at Venetian, I would say this was 'just as good' a room and hotel, although the canals and shops connected to Venetian are much better than MGM.