As far as cavs go, my friends with them have all had heart problems in their dogs. That doesn't mean that they died young though. Most I know lived quite a while (the oldest was 14) but they still had heart problems for at least the second half of their lives.
I just watched a friend put an ADCH (agility dog champion) on a Cavalier today. Health issues and all, if I wanted a small dog, it's the first breed I'd go to. I have never met one that didn't have a phenomenal temperament, and while the health issues are certainly huge, going with the right breeder is going to stack the deck in your favor for that. If I remember correctly, the breeder I know said their dogs were averaging 11 years at this point, and they continue to work to improve it.