I'm not sure you need an exterior service for this, but the concept is something that should be a definite feature for each app you build. With Streaks (streak

I'm not sure you need an exterior service for this, but the concept is something that should be a definite feature for each app you build. With Streaks (streaksapp.com), we ask if the user is enjoying the app. If yes, we ask for a rating/review. If not, we direct them to send feedback (email, Twitter, etc). In Hexiled (hexiledgame.com), we don't ask for a review/rating unless the player has successfully beaten the level, assuming they'd be feeling more positive about the game. In US and AU, that's helped us get 1,000+ reviews averaging 4.5/5. We also put our names and faces in there so people realise this is a first-ever mobile game by two guys and not a big-company effort where your meagre IAP will get lost in the millions made. I noticed that with Highball they try to put the app in the context of "This is a free thing we have made, go easy on us" very early in the on-boarding. Otherwise reviewers can be absolutely brutal and often unfair.