My 2 cents:
Option 1: Go ahead and buy a Wal Mart bike. Keep it a 1/2 a year, 12 months or even longer. After that time you will have a better idea of what you would like in a new bike, if the break downs haven't frustrated you from continuing to regularly ride a bike. The component quality of this type of bike really is not good, and if you are regularly riding, like everyday, you will break something eventually. A bike is better than no bike, though.
Option 2: Places like Performance offer low end bikes of a better quality at about the same price, and can help guide you as to proper bike fit, which will greatly increase your cycling enjoyment. Even if you pick option 1 or 3, you should at least visit a bike shop to see what a bike properly fit feels like. You might be able to make a slightly too small bike work, albeit uncomfortably, but you do not want too large a bike. They can also make some recommendations that you may have not considered. Every shop offers a test ride, they usually require that you leave your license as a security deposit. Some shops also require you to sign a waiver of liability agreement, most do not. Some shops require the use of a helmet. Some shops will give you a discount on accessories if you buy them at the time of purchase. The shop I bought my latest bike from gave me a 20% discount on all accessories, plus a coupon for a future discounted purchase. Performance gives you points. When I bought my mountain bike through them, I had enough points to get a helmet without additional cost. Salespeople at bike shops are a distinct advantage over Wal Mart in that as cyclists, they possess much more knowledge. Your typical Wal Mart employee will probably give you a bunch of I don't knows. A bike shop salesperson should be willing to share information and help educate you. If they act like they are just trying to get the sale, or are talking down to you, walk out and go somewhere else. Once you have had a positive experience at a bike shop, you will never again go back to a department store bike.
Option 3: Get a used bike. If money is a limiting factor, you can probably get a better used bike than a dept store new bike. Craigslist is a great source, newspaper. It is good if you can visit some shops so you have an idea of what is a good brand, however brands like Mongoose also have low end models in Wal Mart as well. If you are trying to avoid a low end bike, avoid bikes that have really flashy paint schemes-In particular those that you see in Wal Mart as opposed to higher end bikes. You need to know what you are looking for, moreso than in a new bike, so make sure you are educated before you fork over your money.