I second PP - avoid big box bicycles like the plague. They are awful long term. Try bike shops and find a bike you like and then look for it in closeouts either at that same bike shop or a different one. Around here there are bike shops that specialize in selling last years models. Also check online. A quick google search for the model you want could net $100's in savings, even after shipping and assembly. Also check craigslist after you've found what you want. We bought youngest DS a Giant off of craigslist this summer. We paid $100 for a $500 bike. The bike was in very good condition - just a tire that was the wrong color and even after replacing the tire our total cost for the bike was only $140.
Other good brands are as PP mentioned Specialized, Trek, Mongoose and Schwinn. Be very careful with Mongoose and Schwinn though, these two brands are sold both in bike stores and in the Big Box Stores so their bikes run the gamut of crappy to excellent. So research the model carefully before buying.
One thing to think about when buying a bike - the accessories are almost all interchangeable - seats, pedals handle grips. So buy the bike where the frame fits your frame - that's really comfortable when you sit on it and grasp the handlebars and that has really good high end derailers and brakes. Don't buy a bike because the seat is cushy and it has a suspension seat post. The seat is fluff and can very easily be upgraded on the aftermarket. Bike Nashbar is a good source for good seats, etc.
DH has a "commuter"/city style bike and loves it. It's perfect for the bike paths and street but can also handle a small bit of dirt if the bike path happens to be sandy or muddy.