I've had a few... the only one that ever worked properly right from the start was a 20" single speed/coaster brakes. I had a 26" 'mountain bike', but it never shifted properly, and the brakes didn't work too well...

My mom's bike is from walmart, and it actually has held up for several hundred miles of mostly just riding on the road, and it's fine. The stem creaks, but you can't win em all for 100 bucks... and we could probably tighten it.