I used PDO a lot at my last job; it's one of the decisions I regret. Whenever I tried to do anything somewhat advanced, it had a tendency to segfault. Especially bothersome to track were segfaults due to leaving a statement open: at least through 5.1.6, PHP destroys objects at end-of-request in order by creation, and PDO doesn't like having the connection destroyed before its statements.

At my current place, I inherited a badly-designed wrapper class around OO-style mysqli (and stored procedures); the mysqli part seems to be pretty good.