I've owned one bike bought from a WalMart type store (Canadian Tire actually) and it lasted me one night. The fork started going easy rider on me within a few hours. Here's what happened: My parents did not want me riding my much more expensive BMX bike to school which was starting the following Monday. So on the Friday night before we went out and got me a cheapie from Canadian Tire, maybe it was Toys R Us... Anyway when we got home I went out with a friend to get used to it and after a few drops off of curbs and bunny hops up them the fork was done. I stuck some BMX forks in them (Made Wheelies particularly interesting) and still rode the thing around but boy was it a beast and required constant maintenance until the frame finally went snap. Lasted a little more than a month.

I've never known anyone who has been a serious cyclist who has managed to get a bike like that last too long. Not that I know many who have bought bikes from such places. Best advice in this situation is to take a look at the used market. A used bike from a decent bike maker will easily be much stronger and more reliable than one of these bikes.

A good example to follow in this situation is to look at the bikes people are using as beaters on this forum. Are they using new WalMart bikes or picking up old bikes from reputable builders? We all know the answer to that and the really neat thing about that is some of these bikes are 10-15 years old and being used for daily commutes with little fuss.

Edit: Forgot to add WalMart is evil!

But seriously they are.