I've used it, it's okay, but very expensive.
I use it to give an initial boost to my download numbers. That gives you a chance of getting into the trending section in the charts, which then allows your download numbers to grow organically through the store. You end up paying something around £0.15 per install, so for 1000 installs, it will cost you £150. Now this seems like a lot, but if this gets your app trending, then you should see much better growth than without this cost, and you'll also get some money back from in-app ads if you're running them.
It's difficult to say though how much you'll make back, but getting a steady flow of downloads is the most important thing.