I am a beginner as well, and can understand both sides of the argument. If you can go to a bike shop, it is probably best. If you have to buy a dept. store bike, then there is nothing wrong with that. Just check the bike out as good as you can before you buy. Make them get it off of the rack for you so you can see how it feels as best you can. Check everything on the bike (assume that it was put together by someone who didn't really care). As long as you read the owner's manual you should be OK. As far as durability goes, if it should break (and you survive) there shouldn't be any problem returning it. The customer may not always be right, but the customer is always the customer.

Welcome aboard and have fun.