BDD is absolutely terrible. Like several of the other posters, I've also seen just about every game Glen Davis has played since he came into the league. My thoughts:
on defense:
-too short to guard centers. is ALWAYS abused in post-up situations, and can't rebound to save his [dang] life. (think: 2010 finals where Pau destroyed BBD and got about a million offensive rebounds over his fat head, causing us to lose the game)
-too fat and slow to guard anybody else
on offense:
-once upon a time (for about one season) BBD wasn't super terrible on offense because he wasn't afraid to take the ball inside and get a good shot. but then once people figured his game out, he proceeded to completely suck balls by leading the league in % of his shots blocked by the other team (something absurd like 17%. nobody else was even close). after that traumatizing experience, BBD just began jacking open midrange jumpshots. Wouldn't be too terrible, except for the fact that: 1) BBD can't hit this wide open shots at even a respectable rate. it's why he's left open from that spot, just like Rondo 2) long 2's are the least efficient shots in the game.
-which brings us to present day BBD, who is the least efficient scorer on the entire team. this is a fact (excluding total scrubs, of course).
If you're bad on defense, and you're bad on offense, you're officially just a bad player. BBD is clearly below average in both categories, and it's my opinion that just about anybody is an improvement over him. Even players that are considered bad usually have at least one discernible skill that's above average. Glen Davis has none.