I've used it a few times, when I wanted to boost old images that hadn't sold. It seemed to bring them back at the top of the search for a while but didn't boost my sales. For a contributor with only a few photos online and not too much revenue, which is my case, I think it's better to focus on trying to improve your own key wording skills rather than pay for the service.
I didn't use it myself. ther are some programs (freeware) around who do the keyword suggestion for you as a tool and afterwards you can sync those keywords with pictures alike. I know there is one of the major ( some call him the best) stock photographers who offers a free online keywording service. It gives him an insight in the trends and competition around.
Neither have some experience with that service either.
Suppose it has some value while you upload much... and earn much, than it can help: I was already thinking about it, while I rekeyworded my first pictures, but than done it myself. BTW: it was for me quite surprising how few keywords I´ve been able to find out in the beginning and how many in comparation came up few weeks ago:) Its probably just the practice: maybe I will try to use that service sometime, but not now..
It's a great service if maybe English is not your first language or you do not have the time to keyword many files. Another good use would be to use it on files you have keyworded, files that you think the image is good but you have not generated any sales on it, sort of last chance saloon before you bin the image or give it to the free section. Just my thoughts :-)