My keyboard has a broken 5 key, which is not due to being worn out, but to me being an idiot and trying to taking it away and putting it back - the second part of that didn't quite work. It also is a bit dirty (especially under the keys) due to me sometimes eating in front of my laptop.
There are no really worn-out keys though, which is probably because the keyboard being white on black and not vice-versa. There are some keys that are dirtier than others though, for example Space and RShift aren't exactly looking nice anymore, and I use both of them a lot when playing Pokemon on emulator. WASD are looking ok though, which shows that I'm not playing shooters much.
Conclusion: If I'd clean my keyboard, I'd look like new again. I should probably do that. Then again, there's stuff under the key I won't be able to remove, or my other keys will fare just like the poor 5.