Hi Phil, I haven't as yet, however have been considering it for a new product I have released.  On reading on some business facebook groups I am a part of I have noticed that it can be quite hit and miss.

The biggest takeaway I have gotten from my research so far is ensure you have all the right paperwork ie contracts - branding specs etc in place is key.  What are they going to do on your behalf (ie how many posts, what are they going to say, what hashtags etc they are going to use etc)  and How are they going represent you , when , how often etc.

I was shocked to hear how many 'scammers' there are out there - with fake followers, bots etc that will not yeild any results.   

I say Go for it!  but, like all business - Do your own research and Due Diligence!