If you're going to buy a department store bike, I can vouch for the
Forge from Target. I've put around 320 miles on mine so far, and it's great. Looking forward to breaking the $1/mile barrier this spring.
That's a little less than two week's worth of riding... I would hope the bike's intended to last more than a couple of weeks!
You really need to beware of big-box-store bikes. One reason is that to keep the cost low, they use cheap materials, the other is that they're more often not assembled right, to the point they're not safe to ride out of the store.
I live near what's probably the world's worst Wal-Mart anyhow, but here is a sampling of their wares over the past holiday season:
Note the unique configuration of the fork on this bike... this one's from just before Christmas of '08.
Here's a group from a couple of weeks ago... noting that on nearly every bike, the brake levers are installed backwards. Yeah, these are kids' bikes, but would you trust your child with one of these?