He's certainly a top 4 or 5 Defensive Center (Dwight, Varejao, Noah, Perk, Tyson).
Overall? Wow - just on intangibles alone, I'd say he's a top 10.
Stats? Decent stats over the years..he impressed me during the 08-09 playoffs vs CHI..we were without KG and lost Powe early in that series, and Perk manned up quite well.
Perk averaged around 13 pts, 12 rebs and about 3 blks in helping us beat CHI then.
He was our only low post presence for that series due to injuries. Glen played some in the post, but not as much as Perk.
He is not polished offensively, but from time to time I've seen him with a few moves down low.
He played well in the Celtics' System, though....will be interesting to see how he is integrated in OKC. If he gets 30-35 min a game and stays healthy, he could put up some noticeable numbers.