My biggest thing with Maggette, based on the small amount I've seen him play, and maybe Kicky can comment on this, is that He reminds me of Kaman in that to be his efficient self, he needs the ball in his spots, whether it be to the teams detriment or not. The offense stalls when he has the ball. In other words, he can only be effective with the ball in his hands in the half court. He's worthless otherwise.
There's some truth to this, and he's something of a ball stopper once he gets the ball in a few positions (the extreme left side of the free throw line appears to be his favorite spot).
This, by itself, however does not make him completely worthless.
For one, I think when you're at the efficiency level that Maggette is offensively I think you start questioning how much you care that he stops the ball in those situations. Earlier I compared his ability to get to the free throw line to Dwayne Wade; it turns out that this comparison was a disservice to Maggette. Maggette takes 7.9 free throws per game as compared to 12.7 field goals attempted per game. Dwayne Wade attempted 9.1 free throws per game against 19.6 field goals attempted per game. This indicates that Maggette gets fouled at a radically higher rate than Wade. In many ways, this justifies his relatively high usage rate because every time you get fouled to take free throws that counts as a possession "used" for usage rate purposes, and at 83% on free throws it's simply a good use of a possession. This is one of the reasons that Maggette is near the top of the league in TS% (among offensive players with a significant number of shots per the season, i.e. not Fabrico Oberto or Amir Johnson, Maggette is near the top ten and one of the top 5 non-centers) while having a usage rate tied for 32nd in the league. All this despite not having any three-point aspect to his game, which is how Kyle Korver ended up having a high true shooting percentage. His efficiency and usage rates are highly similar to Paul Pierce, which is obviously good company.
The guy gets all those free throws because he is unbelievably strong for his size (he might be the most ripped player in the league).
This allows him to draw a lot of contact and finish hard at the hole; which sort of makes him a bizarro Vince Carter. I've also been to games where this has caused significant problems for the other team with foul trouble in the fourth quarter and no way to stop him down the stretch. This, for example, was a major theme in a late season one point victory against Toronto.
The other side of the coin of his strength is that he can create mis-match problems by being able to credibly defend back to the basket post-players and that is part of the reason that the Warriors actually played well towards the end of the season. Maggette's massive arms and legs allow him to push bigger players off their spots.
He's not a perfect player by any means, but you can really do a lot worse. Frankly I thought he was worth his money this season.