If you've been to any of the Chili's in America or the UK, you'll pretty much know what to expect. It's an American restaurant that does Mexican food. If you don't know what to expect, take a look at the website, http://www.chilis.com/.
Chili's is normally very busy at peak times because it has bottomless drinks, one meal is enough for two Malaysians, and kids eat free, so you may have to wait a while. The American menu has been modified slightly to make it Hallah, but that's not to say they don't serve beer, because they do, and it's pretty cheap by Malaysian standards.
My wife and I particularly like the fajitas, chicken wings, and deep-fried fish. Also, the fries are the big and chunky sort, which you don't get too often in KL.
One word of warning, Chili's isn't too particular about cutlery, and they give you a steak knife and fork for everything. When my friend asked for a spoon, he got a soup spoon, which was less than ideal for eating rice. I can't imagine what would happen if you asked for chopsticks.
Chili's is really a place to hang out with your friends for a while, as the chairs are pretty comfortable and you can gossip with the knowledge your drinks will always be refilled. (You do have to pester the waiter for a refill, mind you.)
P.S. The Chili's in Mid-Valley is nearly identical, but its around the outside of the shopping centre.
P.P.S. If you just want refillable drinks in KLCC, Burger King is cheaper, but the seats are less comfy.