I'm another keyboard/mouse user. I can't imagine that changing any time soon. But if you think a different controller is best for you, then go with it.
All I would say to anyone contemplating getting a different controller is give it a try before you buy - and preferably try it with a favourite FPS game to make sure you can use it effectively.
I mean, I've got an Intellimouse Explorer - but it doesn't feel as comfortable as some other mice I've used. Don't know why, but that's the way it is.
You should choose the most ergonomic device for you, whether it's mouse, keyboard, gamepad, joystick or whatever. I know it's sometimes difficult to try before you buy - but try and find a store where they allow you to do exactly that. You don't have to buy from there (if it's expensive), just try out the goods. And ignore the salesperson breathing down your neck.
There's nothing worse than using something that is not comfortable for prolonged periods, and you could end up with problems as a result.
*cuts up soap box with lightsaber, and exits left*