My psychiatrist sent me for a psych eval. a few years ago because he was having trouble getting me stabilized and was concerned that maybe he was missing something. The testing was done a few hours at a time over several sessions. It was pretty painless.

What the psych eval. did was come up with a diagnosis (actual several in my case). There were no real surprises in the results, but at least there was confirmation of the diagnoses that my psychiatrist had been working under.