I saw this offer as well the other day when I was checking out during an eBay purchase. BillMeLater is completely legit, and if you don't mind a minor hit on your credit report it's a good deal.
A few years ago Buy.com had $10 off any purchase with BML, it was used many times by many CAGs to get thing almost free and no one had any problems with the actual BML service. Amazon adopted it for a while and now it looks like Paypal is trying it out.
It's nothing like a credit card. They mention in their fine print how it works, essentially taking out a mini-loan for the amount of your purchase. If you pay it off when you get the bill, you won't pay any interest and you basically never hear from them again.