The Fountains at Bellagio are definitely worth a visit if you're in vegas. They run every 30 minutes during day-time hours and switch to 15 minute intervals at night. Music is played through speakers conspicuously hidden in light posts along the street and, so far as we could tell, the music selection changes throughout the day.
The fountains themselves are pretty incredible. I had not seen anything like them anywhere else. The shows do tend to draw crowds no matter what time you happen to walk by, but you could probably get a good view at any time. As previous reviewers have mentioned, I'd imagine the view from a room at the Bellagio is quite fantastic.
There aren't a lot of *free* attractions in Las Vegas and I'd highly recommend adding this one to your itinerary.