As a Jacksonville fan, I had the unfortunate opportunity to watch Leftwich on a weekly basis while he was here.
He takes a lot of sacks and has a lot of incompletions. The Jacksonville line isn't an elite unit, but it's certainly not a bad unit in pass blocking. Leftwich holds on to the football MUCH too long, much like Harrington, David Carr, and those other quarterbacks who have loads of sacks. He has the pocket mobility of a statue and isn't able to evade pass-rushing defenders. On top of that, his throwing motion is very long and slow, and he doesn't look off defenders very well. That may not have been a huge drawback at Marshall where he wasn't playing against elite defenses, but in the NFL it is a big liability. Defensive linemen are able to see when he's going to throw the ball and bat it down at the line of scrimmage; safeties are able to read where he's going to throw the ball and move to that side of the field; cornerbacks are able to predict when the ball will be thrown their way and move to deflect or intercept it.
Leftwich isn't all bad. He's got great arm strength; his accuracy on the deep ball isn't great, but it's acceptable. He'd probably be an acceptable backup on a team with a couple of good WRs - I could see him making some plays in Cincinatti, for example. I definitely don't think he'll do any better than Harrington in Atlanta.