i have an i/o, not had it long so i can't tell you from my experience, but there are still a bunch of guys here that use them, a friend of mine just changed his, it had over 11k hours on it (volvo sx), i know another guy with a bravo ii that is an '87 (i think) and it has outlasted 3 350s and that's not uncommon. you have your maintainence stuff, bellows, ujoints and gimble bearings, but they are pretty easily changed with very little mechanical ability.
that being said if it has a mercruiser alpha drive, stay very very far away, the volvo sx drives, and mercruiser bravo II drives are pretty bullet proof.
i have another friend with an alpha drive, he tears up two a year.