I recently started using it in one of my sites for a test run. I had no problems with the set up and the site works fine under it. I like the idea that it saves me some bandwidth, especially if it's spam traffic. According to its statistics about fifth of my traffic is spam.
There is 138 visitors so far who were shown a captcha instead of being blocked straight away. 16 of them solved the captcha and left a message. A lot of these messages seemed to be weird/spammy types like "this is a good site" or just "hi", but some were messages of frustration. One message was "what the hell are you doing? for enter to your site must send request for what? are you crazy.", and if it was a legit visitor who was challenged by a captcha to access the site I can definitely understand the frustration..
The possibility that legit visitors might be getting blocked or challenged is why I'm not sure if I'm gonna keep this service, especially if it doesn't really prevent a lot of spam. On the other hand the number of legit blocks seems like a small percentage of the overall blocks or challenges, and you can manually mark an IP as trusted. Overall, I think it might be worth a try and see what you get. If it significantly prevents the amount of spam you get and saves you a lot of bandwidth, a trade off of alienating a few random users might be worth it.
Btw, we discussed CloudFlare a bit in this thread as well.