I got one of the inexpensive REI 29 MTBs and it is great on the streets. Good brakes, go over anything, don't worry about sewer grates, and it can turn a good rate of speed. Knobbies aren't the best, but when you run into leaves and mud, like all this crap from the storm over the last few days it doesn't hurt.

Plus, it is a great winter bike.

You will not regret getting a 29er. If you can swing hydraulic discs, it is worth the money - less fussing with them, thats the only thing I would change.

And you are extremely unlikely to get a flat....