I have done this twice, but not for free, for a discount. My first experience was awesome, and I linked to that post in my shop and a couple of listings. I don't know if anyone has actually ever read it, but it's a solid review which I appreciate as I struggle with photos and think my work is much better in "real life". That transaction was a great experience!
My second blog review for discount was less than stellar - basically the blogger copied exact phrases from the first blog post and didn't add anything original. :( It hadn't even occurred to me that someone would do that! I can't use that blog in my shop, and I haven't promoted it, because I can't support the copying!
The upside is that the blog mentions provide backlinks to your shop that will aid in google searches. I have seen a decent number of google search results in my stats, especially since blog post 2, although I can't say for sure that there is a connection!
Should I do this in the future I'll check out other reviews written by the blogger more carefully, ask if they always provide original content, and ask if they'll promote - my first blogger did an awesome job promoting the post (and me) across lots of social media, and I did see stat action from that for sure! In fact - it led to a sale :)