Well, a lot has been said here and I'd like to respond to a few things because I don't really see and understand this state of panic.
For one, regarding this whole being forced to Android thing: I have yet to see an Android phone that can scroll something as simple as the Settings App smoothly.
Regarding this whole Game Center competitor, let's not forget that Google does a lot of experimentation and while being able to save state in the Cloud and have a unified way for Achievements and Leaderboards, let's not forget that Game Center is so much more from a developer's perspective.
It gives you fantastic APIs for turn based or real time multiplayer, based on various P2P or P2P and mixed server architectures. All of this service is given to you for free, most small devs will not require to host any game servers whatsoever. It also spares them to actually get down to the nitty gritty of low level network programming, sockets and whatnot. It gives you real time voice chat capabilities, again without any servers whatsoever. It also gives you matchmaking, queue estimation and many, many other things.
All in All, Game Center is a fantastic set of APIs and consists of much more than most users see. It enables developers to produce great results easily and quickly without requiring any kind of investment into own architecture. This is a huge deal and goes far beyond save games in the cloud.
As so often, we will have to see how Google plays this, however from experience I am quite confident when I say that Google's implementations generally lack from an API perspective. This goes across all Android APIs and when you know iOS counterparts the amount of lacking features you have to implement yourself, the general extremely "manual" feel of Google's APIs is just mind boggling. This is also very curious because Google does experiment a lot, change a lot, invents new stuff with its APIs which is arguably even useful, however still recommends developers to follow rather than actually improving its APIs.
My bottom line is: While this is something that has been lacking on Android, let's not forget that there is so much more to Game Center than that green felt app with leader boards. It's a comprehensive set of APIs that actually enable indie devs to produce the kind of multiplayer games we have on iOS now.
On a side note, it remains to be seen how widely this is going to be adopted. Android devs are certainly going to be happy with it. However, from what I could read about this already, neither voice chat nor real time multiplayer will be supported on iOS or on the web. Meaning, there is little cross platform to it, despite achievements and save games. I'm not even getting into the discussion of how frequent the use case of having an iOS phone but an Android tablet actually is. Since multiplayer gaming itself however won't be cross platform, developers without own infrastructure will still need to leverage both APIs on their respective platforms.