It all depends on what you're using the bike for. If you're just going to ride the bike down the street every once in awhile to pick up something from the store, or for short weekend pleasure rides, a Wal-Mart bike might be the best for you. They're not built to last and the components aren't great, and it's not put together as well as a Trek might be, but then again, you don't really need a perfect machine if you're not going to ride it very often.

If you're going to be putting a lot of miles on the bike (I would say, as a totally arbitrary number, 1000 miles or more in a year), I would recommend spending a little more money to get something that's quality. You can pick up a decent, lightly-used vintage Trek or Fuji for $150-200 on ebay or Craigslist and pay your local bike shop to give it a good tuneup, and you'll still be going cheaper than buying most new professionally-made bicycles.