I recently added Appirater in an update to one of my apps and I believe has really helped gather positive reviews.

Getting the timing right is a bit tricky - I've opted to prompt the user after ten uses at a point where interaction with the app won't be obstructed by the alert.

As a side note, I think that the alerts should be shown only once. I was using another app that would prompt every other time to rate and the default dismiss button was "Remind me later"...very annoying.