My dad, an airplane mechanic, has the saying "No-one can afford a cheap airplane".
The Wal-mart bikes have three main issues - safety, maintenance, and poor riding characteristics. You'll go slower, struggle more, break down more often, spend more on strange proprietary parts made out of cheap plastic, and have to worry about the brakes failing as you're driving in traffic, or the pedal snapping as you go to power up a hill.
If that's acceptable to you then go for it. I know that here, a tune-up runs about $40-$50, which needs to be added to the price of a chain store BSO.