I recently tried Keen.io for a project I'm working on, and it didn't work out - the metrics I was looking for were too complex (eg. the average lag between signup and the third time a user does a particular action) to do natively, and the suggestions for how to get that working in Keen (doing some calculations on the event data before sending out) I wasn't interested in.

Having said that, the guys at Keen were great to work with as I was sorting this out. The email I sent to their generic "contact us" address with some data modelling questions was answered promptly by one of the founders with solutions which, while I didn't like them, were interesting and would have worked out. They were also generous in letting me crash through the initial free quota while I was figuring out if the thing worked out for my use.

So while I didn't sign up as a paying customer with them (and I've stopped sending them events), it was among the most pleasant interactions I've had with a prospective analytics vendor in a long while - well done guys!