We bought our monorail passes before we got to Vegas and saved a few $s this way. I would suggest checking out prices on their website before you get there. Although you can purchase them easily enough once there, it was nice just to have the passes in advance.
We took the city bus one day and this was not an ideal situation for us. When we first got on downtown, the bus was not that full, but by the time we got midstrip, we were all packed in there like sardines. We did not really enjoy the bus ride at all.
The monorail is pretty quick and offers a quick non-crowded way to get around. I think that it is most useful from getting from one end of the strip to the other; however, if you are midstrip already, most places are just as easy to walk to. You have to walk to the back of the casinos to get to the monorail and that probably took as long as it did to get to the back monorail areas in some instances.
There are also free trams running down by MGM/Luxor/Excalibur, which we liked and used on more than one occasion.
Overall, I think it is a good deal if you are really going to be going a far distance, but otherwise I would suggest walking the strip and taking in the sites. The monorail tends to run in the back of the hotels and you don't get to see as much from this viewpoint.
This review is the subjective opinion of a TripAdvisor member and not of TripAdvisor LLC.