I have a wal-mart bike, the 29" genesis mtb. Believe me before i boughtr the bike for my build i looked everywhere( ebay, amazon, online bike stores, local bike shops) and found for the money and quality i really could not find better. Really any thing better was $450 up. And i couldnt see trying to modify a over 500$ bike. Maby i just got lucky and got a good bike. If you feel like taking a look at it go to my profile and check it out.
When it comes down to it you just have to go with what you can afford i guess..