I see the Fountains of Bellagio every time I'm in Las Vegas.
What can I say about it? It is free. The fountain water "dances" to the music to some degree. Depending on the time of day, different songs will play.
I heard "Viva Las Vegas" in a recent trip.
A lot of people will stop and wait for the show to start but generally you can find a spot close to the front to take pictures/videos. The show happens generally every 30 minutes during the daytime. There are some people peddling their wares in the area and with any busy area, you need to be careful of pickpockets (although I generally feel safe when I'm on the Strip and nothing generally has ever happened to me beyond one or two overly aggressive buskers).
Would I come to Las Vegas to see this? No. But if in Las Vegas, it is definitely worth seeing once and since it is free and happens frequently/regularly, it isn't something which is difficult to see.