Are OpenFeint and Plus+ still out there, let alone being used?
One thing I liked about OpenFeint was it recorded achievements when u didn't have internet access (played many games on IpT away from home where there often was NOT wifi). The logo eating up a whole corner in the app icon was annoying, but I guess unlike others, I've learned to accept it. Don't remember much about Plus+. I don't really use Game Center much, even though it's been baked in since... ios5 IIRC? I used to use it to play MP games of a card game (Ascension), but the dev last year forgone that for their own solution. IIRC, they had to do something like this when they released their game for Android and Steam, and it may even be cross-platform now too.