I put 1000 miles + on a "Wal-Mart" bike that was well-used when I bought it at a pawnshop. Other than getting a wheel trued and adjusting the brakes and shifters, I performed virtually no service on it during those 1000 trouble-free miles. SInce then, I had the frame powdercoated and pin-striped, and upgraded some components including the rear shock with the help of e-bay and swap meets. (Horror if horrors, its not only a "Wal-Mart bike" but a full suspension.) I have since built four CF bikes and have a total fleet of seven, but I rode the Wal-Mart Schwinn a few times during the last two weeks and was reminded of how much I like it for care-free comfortable urban riding through potholes, bad roads, over curbs etc.
My time on my 15 mile ride to work on this "Wal-Mart" FS bike the other day was only about 4% short of my best time on the CF road bike.
If you buy a non LBS bike it helps to be your own mechanic and you should go through all adjustments and check bolts when you take it home.
Don in Austin