I had some nine year old Bose speakers in a car. A couple of them sounded okay, while the others were trashed. The problem you can run into with Bose is that the speakers themselves are not the sturdiest things. They are cardboard cones with paper surrounds (not as crappy as most stock speakers, but not a whole lot better). When in good condition they can have decent tonal qualities, but they are pretty easy to destroy, especially once they get a little older.
Most Bose speakers are 2 ohm, so if you are going to replace them you'll need to find a 2 ohm replacement like Infinity Kappas (assuming yours are in fact 2 ohm). As far as whether or not replace them: you just have to decide if they still sound okay. You won't get anything too great for $60 though. Visit some car audio stores and listen to some speakers. If you like them better than the Bose, then you have your answer.
Wow, that was long winded.