I used BillMeLater several years ago to make a couple of large purchases. I specifically used it with the intention of paying off the balance before the 6 month grace period ended. So there was no interest. (I don't know if they still do that.) I was very pleased with it. It let me spread out the cost of a $900 bandsaw over 6 months without any additional cost. That's a no-brainer!
I would still use them again today, but I am sure they wouldn't let me. That's because I walked away from a $450 charge because they refused to properly investigate a claim I filed against a fraudulent camera seller. (The seller was fraudulent, not BillMeLater.)
When I told them that a significant portion of the charges were incorrect (and criminally fraudulent), they blew it off and refused to investigate. However, to their credit, when I notified them in writing that if they didn't investigate and correct the balance, I would not make any payments at all. To my surprise, they wrote off the whole thing, and I never saw another bill after that. (I was willing to pay what I rightfully owed, just not the fraudulent portion.)