I've used it a few times, and didn't really have any problems. It is basically a small line of credit. The first time I used it, it gave me 3 months to pay the bill without interest, which was handy because I needed the computer parts but didn't have the cash at the time (years ago). The second time I used it because the website didn't want to authorize my Visa bank card for some reason, but I also got $10 off the purchase and no interest charges so hey. The third time was because the prior purchase that gave me $10 also gave me another $10 credit, so I went ahead and used that.
Their interest rates are worse than a credit card usually, but they often run specials or deals with websites. And like credit cards they have a grace period, so if you pay off the balance within the period you don't get charged any interest or fees.
All in all, no bad experiences really so long as you treat it like it is - an unsecured high interest line of credit that can bite you if you let it carry a balance.