I've lived in Vegas before the Bellagio was built, and I worked at Bellagio--Let me tell you, the fountain show is stunning and is still one of my faves in Vegas.
The fountains are synchronized to music, and is a spectacular show that is FREE. Granted you can't opt out from the weirdos standing next to you, but like I said it's FREE so it's bound to attract some eebie jeebies or snot nosed lil' kids.
I believe they start at 3pm, every half hour, from 8pm-midnight every quarter hour.
I highly recommend dining at Olives (deliciousness!) and sitting on their patio, or having a cocktail at Hyde to watch the fountains. Much better than viewing it from the strip, and you can get loaded while doing so!!! Cheers kids!