He's one of those guys who probably won't kill your team to start, but he won't shatter records either.
A lot of middle of the road QB's on good teams are in the same vein.
I'd say he's not one of the top options unless the Bears are playing a poor pass defense. He's pretty much on the level that Plummer was at last year...10-12th best QB stats, maybe one or two huge games here and there, but generally mediocre stats each game like 200-250 yards passing, 1-2 TD's, and 0-1 INT's.
The best part is the low INT totals...if the Bears are winning all the time, they don't need to throw as much which will limit the amount of risks Rex will have to take. That's the advantage of having a QB from a good team.