As Momentum just mentioned you do get what you pay for. Schwinn unfortunately is no longer the quality bikes they used to be. they went out of business a few years ago and someone bought them and the bikes the produce are "walmart quality." A "walmart" bike isn't neccessarily bad, but the components are meant to be going 10-15 mph. Which means they will get hot and do things that can cause you to crash. As far as the frame goes, they are good if they have been properly welded together. There is a schwinn cruiser i've seen a lot of people on here use and really enjoy it. I know from my own experience that paying a little more made a huge difference in the ride of the bike.
My advice is, if you do choose to go with a walmart bike, make sure you upgrade the hubs and breaks on it. You can by after market wheels from your LBS. or get them off of an old Trek MTB (or something like it) at a yardsale. Or buy hubs from any of the parts dealers on this site. Also plan what the build is going to be.
I used a Trek Cruiser Classic for my build because I wanted a certain look. The bike cost $250. It was a hard build because I had to do a lot of mods to almost everything on it. But in the end it was worth every penny and mod i did. I don't recomend the bike for a first time builder though, unless your great at problem solving and very machanically inclined.
Quality bikes are almost anything you'll find at your Local bike shop instead of walmart. But a lot of people on here a very happy with walmart bikes. So in the end it come down to planning your build and how much you want to budget to that build. And as momentum said, "you get what you pay for."
hope this helps
Fred