I've been a member of BoardProspects.com since Dec 2016. It has been a big dud. They have averaged one board opening a month and most of those are in NFP and start ups. I met company reps in person as the company is Boston-based (as am I). They claim to place many board members behind the scenes through unadvertised positions. While their interaction and sales tactics seem far superior to ExecRank, I would say their results are similar. Since I joined, I believe BoardProspects has changed their business model in that you can post your resume for free, but you pay $199/year for a premium membership that allows you to apply directly for openings. In that respect, you can save your $199 until you see a position that actually interests you.
I've been getting the same email marketing from BoardProspects. They also, tried to schedule a phone call with me to convince me to join them, I decline the meeting because I am not sure of the value they provide. ExecRank did the same thing, but I did take the call with them and left unconvinced of their story as well. I hope this helps.