Being optimistic, let's hope that all of the time off has allowed his back to heal or at least get stronger.
T-Mac is just one of those players that can make the exceptional look easy. I think the Knicks as a whole will benefit from have a clear focal point for the offense. He's a very complete player offensively with both the ability to score and create for others, and at least earlier in his career, was a pretty competent defender.
At least on offense, he could fill the gaps and help get the ball to the right folks when they're open. I think Gallo in particular could benefit from knowing that his teammate will reward him with a great pass if he gets to the right spot. Duhon has a bit of tunnel vision to David Lee - and Al Harrington doesn't pass to anyone.
I think Chandler could learn from T-Mac along the lines of the way Ariza evolved into more than just an energy guy with Kobe, much in the way Richard Jefferson evolved into a more complete player by playing with Kidd and Carter. (yes, sure he's devolved...)
Our other acquisition - Sergio will help make our 3pt shooters more proficient and space the floor better with his ability to get to the basket. He will have to evolve a little bit as a scorer in order for his drives to be more effective. T Mac should be a calming presence on the team as a competent ball handler at the 2/3 positions.
Overall, I'm pretty excited about T-Mac. If we're able to add Bosh, while keeping T-Mac and Lee, we should be primed for a pretty solid team. We're still short of a defensive player that could shoot the 3.
Granted, Lebron would rock the Knicks' world... but I'd hedge my bed. We'd have a pretty odd team full of 6'8-6'10" wings, no shot blocker, and no PG with shooting ability (unless that's House... or Tony Douglas). We'd also need to acquire a bruising big man who could hurt people...