Jermaine is definitely overpaid - $20M for anyone not named LeBron, Kobe, etc. is too much. Nonetheless, if he actually is healthy and motivated this season, he will help the team tremendously.
In my view, there's two kinds of overpaid players. There's overpaid players like Kapono, Iguodala, etc. - who get a bit more money than they are worth, but nonetheless they are valuable players, they play a lot of games a season, and are a key part of their team.
Then there's overpaid players who are simply bums who lucked into a big contract - the Jerome James, Stephon Marbury (lately), Adonal Foyle, Raef Lafrentz crowd, who don't even play in games anymore, or if they do are just god awful.
I don't mind a couple overpaid players on a team if they are at least valuable contributors to a team. After all, there's only 5 guys on the court at one time, so overpaying a useful rotation player is often not THAT bad. What drags teams down are the albatross contracts of players who no longer play. That's just wasted money.