I recently had to purchase a new keyboard for my iMac. The keys were getting sticky, and were less responsive. I type alot on the keyboard, and expect a fluid soft touch that reacts to the touch. My new keyboard is exactly like the old one, but works better. I have had the keyboard for over a year, so warranty was out, of course. But I have had keyboards from cheap PC manufacturers last for 3-4 years. Not a huge big deal, but I would expect it to last a bit longer since Apple hardware comes at a premium price. Of course, maybe I just over work the keys. I don't have a very soft touch, but don't bang on it either.
Anyway, at least I was able to get one of the older style keyboards, that match my iMac, I really dislike the new aluminum cased flat keyboard.
