I don't think Turner gets enough credit. Everyone seems to love that we went on a run late last season, but everyone also seems to dismiss how important Turner was to that run. He was putting up Rondo-esque numbers at the point-forward position. Second half of the season he averaged 11 points, 6 rebounds, 7 assists and 1.2 steals... He was arguably the best player on our team. He's a versatile player. I actually think he might be a borderline starter.
Turner can do a few things well. He is a decent isolation player. He has an OK midrange pullup shot. He can run the pick and roll fairly well.
He's a mediocre defender, a poor shooter, and he turns the ball over a fair amount. His rebounding is good as a guard but fairly poor as a wing.
I look at him as a backup point guard, kind of like Shaun Livingston. In that role, I think he's an asset. But you have to be willing to give him the keys to the offense while he's on the floor.
For the Celts, he's a decent starting option so long as the other guys in the starting lineup are weak ball-handlers and the team's best ball-handler is a bench scorer.
All of that said, the fact that Turner was important to the team doesn't mean he's particularly good. On a team that has a legitimate starting point guard, he's not worth playing more than 16-18 minutes a night.