I was hoping someone would gives some answers to this, because I'm on the fence with this also.
I went with MySqli because it's easy to convert from MySql.
I have also read that PDO in php is buggy.
But PDO is the standard(or so I have been told) and I shall go to it as soon as I hear that it is not buggy. Also, PDO is object orientated so getting things from the db and into an object is easier. CLARIFY: I don't use objects to often so this statement is from hearsay, but will start when I learn PDO.
That's my 2cents. Hope others will post here.