Ahh, the staple of the Las Vegas dining experience. If you don't hit the buffet at least one time during your trip, you haven't truly been to Vegas! Back when Vegas was just gambling and cheap food, the buffet was a chance to cut down on wait staff, keep the clientele well fed, and get them back on the casino floor as quickly as possible. Now, Las Vegas buffets are a little more upscale, and while deals are still to be had, you can get just as fancy as you want. Some of the benefits--you don't have to wait for everyone to order, and everyone can get exactly what, and as much of, what they want.
Having tested many of the buffets in town (my wife would say too many!), the general rule of thumb is the buffet mirrors the hotel. If you go into an upscale hotel, the buffet will have better food. A case in point is the Bellagio, which has fantastic fine dining, so the buffet has to keep up. They are definitely one of the best in Las Vegas, but more pricey. Mandalay Bay has a fine array of restaurants, but the Bay Side Buffet at the Mandalay Bay is killer. The same with the Ceasers Palace Buffe, which is one of my favorites.
Bigger doesn't always mean better, but the Carnival Buffet ($22.90 for dinner) at the Rio has tons of stations with every type of food. However, I prefer the Village Seafood Buffet, my favorite in town, since I'm a seafood lover; worth the $34.99. If you love French food, Le Village at the Paris has a great selection of French cuisine--$24.95 for dinner.
One caveat--if you are planning a buffet run, check the hours. Not everyone has breakfast and lunch. Another--pace yourself. Nothing ruins a good buffet like getting full too fast. You don't want to roll around Las Vegas like a hibernating bear because of a dining mistake.
For more on buffets, check www.lasvegas.com and www.a2zlasvegas.com for reviews of all the top LV Buffets.