I think the Walmart bikes several years ago were better and longer lasting. My old bike is a walmart bike. It certainly has some problems, but I've had it for 10 years, and it's still perfectly rideable. I never encountered anything I couldn't fix. It has well over 1000 miles on it, but I don't really have any idea how much exactly.

Today's Walmart bikes definitely seem cheaper. There is more plastic, more springs, etc. With slightly aggressive maintenance and a little bit of extra patience, I see nothing wrong with riding one. I think one will last you as long as you need it to. It's just a matter of what you value more - your limited supply of money, or some slight inconvenience. If you don't have much money, I completely understand buying a Walmart bike.