I released a game on the Skiller platform a few weeks ago, R:P:S Academy. I made a pretty good relationship with them and they were very helpful throughout the development process, and I was helpful to them as well. Their API is still fairly new, just a few published apps use it, I believe mine was the first. I chose the game I did, rock-paper-scissors, entirely for the purpose of testing out their API. I am currently planning on incorporating their API into some of my future games. I also would like a simple easy to use minimalist global high score API for some of my other games.
There are a few things that aren't perfect, but they seem to be doing a very good job of improving quickly and are very responsive to developer feedback. It is a bit diffeernt than a global high-score board. I didn't get much into the single palyer API, but it is geared towards tournament play.
My game uses their turn-based multi-player API. It is straightforward and easy to use, and things like achievements were super easy, as were in game purchases. There is a global leader board based one ELO rating from multiplayer games and they focus a bit on social aspects. Multi-player games are played for Skiller coins - in game currency that can be used to buy in game items or to upgrade your Avatar. They also have an ad service, but there is virtually no developer control or feedback from it yet.
You can check out my Skiller powered game R:P:S Academy: https://market.android.com/details?id=c ... rch_result
It hasn't gotten much traction yet so random multi-player opponents are rare, but you can alwasy play a friend.
You can also download their Tic-Tac-Toe Online game to see the app that the sample SDK is built about.
Also I would be happy to answer any question about my experience with Skiller, and I bet they would be happy to hear from interested developers as well. Just shoot me an IM or eMail if you have any specific question.