My teenage granddaughter's uncle purchased a Wal-Mart bike for her last summer. The bike's brakes were not installed properly and she crashed into the side of a car in a driveway going down a steep hill as she was on her way to work. Luckily she only ended up with some road rash and bruises. If you must buy a bike from a big box store make sure to look it over or have it looked over by someone who knows about bikes before you ride . Probably good advice where ever you buy a bike!
Hubby and I each own bike shop bikes and enjoy them (Bianchi Veloce for me, EZ Racer Goldrush recumbent for hubby.) But, last year we spent several weeks in Hawaii and bought Wal-Mart mountian bikes to use while we were there. These worked for our needs there mostly because hubby is a pretty good wrench and was able to make adjustments which were needed often. We were very glad to get back on our 'good' bikes when we got home from vacation.