Positives are that the bikes come in different sizes, unlike department stores. Probably better quality parts (you'll get a cassette instead of a freewheel?).

IMO, the big negative is that you're buying a bike unseen and it can't be easily returned. You can't beat Walmart's 90-day return policy. Just return the bike to your nearest store in decent quality, with all the stickers, packaging and tags, and you get your money back.