The click wheel is a very good interface, especially when you are very often setting a scalar value (volume, temperature, fan speed, etc) or navigating a linear list (songs, contact names, call history, etc) and using one hand without looking at that hand. So it's a good fit for both MP3 players and cars.

Not so good for typing, but can be quite good when there's a good UI that narrows down options for you at each entered letter.