I've used it for the past couple of years and it's been ok. My thoughts are:
It generally pays for itself every month but you do have to sift through the crap enquiries to find the good ones.
I do mine myself but it does take some setting up correctly. A few years back Google used to help you through it, I had my own advisor for a couple of weeks, for free, and that helped a lot. Not sure if they do that now.
Once it's set up properly, it is easy to use and you can turn it on or off whenever you want.
You need to get your budget right, your key words right and most importantly your location right or you'll be p11ssing money down the drain like no-ones business.
I'll probably start using it again in March, see how I feel.