I think that Carmelo has some character flaws that not everyone understands but he has never been a bad teammate. He won that championship at Syracuse, and has been often regarded as Team USA's best player with the international rules in effect.
A lot of what bothers people about Carmelo I think is a cultural thing, where he grew up in Baltimore is essentially where the Wire took place, and I know that the show is a body of fiction (a wonderful one at that) but its most certainly rooted in truth. The stop snitching video he was in, the cheap shots on the court, while not excusable are much more understandable when you take a look at where has come from. (The drinking and driving thing was straight dumb, but he was in an epic fight with his girl, LaLa, who judging by that video of the Mavericks stands is not the most agreeable woman. And I think we've all done some stupid things in reaction to a loved one.)
Chauncey coming in has done wonders for him but I never thought he was a bad guy or even a bad guy to play with, he just didn't really understand what NBA basketball was like. Kevin Durant went through a similar thing in his rookie season in terms of shot selection, granted he figured it out much sooner than Carmelo did.
When Carmelo hits the offensive boards and commits to defense he's as good as anyone. I think the Pierce comparisons are quite apt in that regard.