Extract from Complaint driven development from Codinghorror.com
"As Steve Krug says in Don't Make Me Think: You don't need to find all the problems. In fact, you'll never find all of the problems in anything you test. And it wouldn't help if you did, because of this fact: You can find more problems in half a day than you can fix in a month. You'll always find more problems than you have the resources to fix, so it's very important that you focus on fixing the most serious ones first. And three users are very likely to encounter many of the most significant problems related to the tasks that you're testing."

And from Version 1 sucks but ship it anyway 
"Instead of spending three months fixing up this version in a sterile, isolated lab, you could be spending that same three month period listening to feedback from real live, honest-to-god, annoyingdedicated users of your software. Not the software as you imagined it, and the users as you imagined them, but as they exist in the real world. You can turn around and use that directed, real world feedback to not only fix all the sucky parts of version 1, but spend your whole development budget more efficiently, predicated on hard usage data from your users."