I've personally only invested in a couple of loans, but even with my little experience it is pretty obvious to tell which loans are/aren't legit. If you only loan out your BTC to people with good scores and a solid loaning history, you are unlikely to get scammed. The accounts without a profile picture that offer insane interest rates are going to default every time, end of discussion. However, if you stick to the safe loans, you can easily profit.
Well a couple of loans doesn't give you much experience. I've now had "A" loans that defaulted. That ratings are not very useful. I did find a few borrowers that always pay and started investing heavy in them. Because of that I'll get a small profit out but it is taking forever to get it out of the site.
EDIT: It is common for scammers there to have a series of loans that were paid off quickly and then to just default on a larger loan and run off. If you follow the advice of the site, you will only lose a small amount per scammer, however, that also means that it would cost you more to collect the debt than the debt is worth when you finally do get an arbitration award.