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.
I have no issues with the I/O. Everything has maintenance. It looks better than an outboard and performs better than an inboard. the Alpha legs are cheap too. They are easy to trailer, if you have to, and you can beach the boat if you are doing the family thing.
IO more efficient than a straight inboard, and no motor hanging/obstruction off the transom. I have 2 outboard boats and an IO. Both serve their purpose. I maintain my boats and never had a problem with either after several years of use. Good luck with your decision...