If I had $3500 in my marketing budget I wouldn't spend it on a service like that. You'd be better off trying other free providers and seeing how you fare with them first. We went with AppGratis who did pretty well for us. It was a free service and all we had to do was make our app free for a day. We ended up with around 35k downloads and even ended up with the number 1 free app in both Spain and Brazil (for a day). It boosted sales for the following couple of weeks but it has since settled back to normal. Nonetheless, I would recommend them.