I got the same mail from app samurai just after I shared my app on reddit. The mail was about their black friday campaign and I participated in it. Also, I told my friend about this campaign and he participated in it as well, and he got the second prize with $1000 ad credit. as far as I know he used that prize.
I didn't use their services yet, by the way. it seems that they are new and they are launching these kind of campaigns to promote themselves. also they are fairly active on twitter (you can find them with typing app samurai)