I tried them out quite a bit around when they first came out and wasn't at all impressed! I agree with Boulder above - if you do try them, make sure you use as specific keywords as you can get and you'll get the best results. I just don't think people click on them. I did not get one sale from mine and ran them for a month with lots of variations to test them out.
If you do a search on the Etsy Business Team, you'll find that very few people have had good results from them. Personally I'm surprised they still offer them for all the "bad press" they've gotten, but apparently people are still using them so they'll keep offering them! :)
jesi