Any bike for that price at Amazon is like buying a bike at Walmart. The thing with internet shopping for bikes is that you don't get to get 'fitted' in it. Without fit, you will loose efficiency. If you pay for that price, don't expect it to be lifetime guaranteed. You will most likely be walking the bike to a bike shop. There wouldn't be better quality that from a discount/department store than from building yourself because there isn't a change of quality in the components. Both the Mongoose and Schwinn are from the same industry (Dorel Industries), so there isn't much of significance between the companies.
If you are just taking a few mile rides, then just get it. If the bike doesn't fit, I would rather not get the bike even if it was free.
Don't expect it do be a reliable bike. It'll take you where you need to go but you will/might run into some problems.