I would definitely use a service like this. The last time a launched a side project I found it extremely difficult to engage users, even users that had signed up with an email. They simply wouldn't respond to anything; feedback requests, surveys, nothing. I took this as a strong signal there was no market or that I'd done something horribly wrong in the implementation. If I could've just paid $X to get some honest feedback, would have saved me a lot of time.