The first time I went to Vegas, I gained 6 pounds in 3 days from buffet over-indulgence. I've since learned to moderate a bit, but I still love some of the Vegas buffets (Wynn, Bellagio, Aria & Mirage). I love the fact that you don't have to decide what to eat from a written description and risk being disappointed when someone else at your table orders something better. I always try a bit (i.e. a spoonful) or anything that looks good and then go back for more of the things I really liked.
I also like the fact that you can mix things up and make your own creations (such as visiting the dessert station to put caramel sauce and pecans on on your breakfast waffle (don't forget to add crispy bacon!).
However, not all buffets are created equal and I won't go to a crappy buffet just for the quantity. These can usually be judged by the low price and waist size of the average customers. I also generally avoid anything with special children's prices since that brings with it a whole range of issues that I'd rather not deal with while dining.