Anyone tried an app review site they got in their junk mail?

When I bought my first domain about 9 years ago, I was amused because I got spam (and even physical letters) offering me a range of domain services, from SEO, to “similarly spelled” domains that were up for sale (at a modest $300!), and even one mob who sends me a letter every single year, telling me my domain is up for renewal and I should pay them instead.

Now that I’ve got my app in the Apple and Android stores, I’m receiving similar messages, offering me free or paid reviews or promotions for my app. Some just want money, some will claim to do it for free, some run “review exchange” sites where you get a review for every one that give.

My guess is that it’s all a load of crap, that there’s no way some random review site can double or triple your sales, but now I’m curious: Has anyone here used one of those sites to get or give reviews? If so, how’d you go? Any noticeable difference in your sales, downloads and / or reviews?