I have used the monorail on 5 separate trips to Vegas. 3 of those trips were for trade shows at the convention center and I stayed at Bally's. Under those conditions I was riding to and from the convention center and at least one way for dinner with friends in the evening. For that set of circumstances it worked great. When I am in Vegas for pleasure I usually stay at Excalibur and the monorail is much less convenient being on the other side of the street and the only reason I used it was folks I was with said that is the method of transportation we would be using. But not everybody enjoys walking no matter what the weather like I do.
Pluses - it is clean, air conditioned, nobody trying to wave sales literature in your face. And they have added trains to cut down the wait.
Negatives - From some casinos it really is faster to walk to your next destination. Some of the stops are hard to find just because of the way the casino and the signage is set up, I have gotten lost several times in MGM. If you are not doing several rides per day it is relatively expensive.