Originally, I thought Game Center was Apple stepping on the toes of companies like Plus+, Crystal and of course, OpenFeint.
Even after I first tried it, I wasn’t overly impressed. It was only when I added a couple of friends that I started to see the huge benefits that it holds.
Cut the rope; a game that I hadn’t played for a while, and hadn’t updated it. I added a friend on Game Center, and it told me that he was ranked higher than me… Not for long! So I beat his score and laughed at him. He proceeded to improve his score and it continued like that for a big part of the weekend (FYI, I’m still ahead of him. 
I’m currently #258 out of everyone.. not that I’m competitive).
Then another friend challenged me to a game via Game Center (I didn’t even realise that Fruit Ninja was multiplayer!)
Basically, within a few hours of looking at it, I was already much more involved and fascinated with it than I had ever been with any of the other aforementioned platforms. However, I think it’s good that game producers are trying to keep their original social option in addition to Apple’s new offering.
It was only relatively recently that I jailbroke my iPhone 3GS (using the jailbreakme.com method), but since then, I’d find it difficult to go back to normal.
I think I’ve mentioned this before, but a lot of people assume that people who jailbreak do so to just steal apps. For the mostpart, this isn’t true. Most jailbreakers just want to be able to fully customise their own property (myself included).
The iOS4 jailbreak allowed me to experiment with different stuff (such as different themes, having 5 icon width, tethering*) but it did cause a few problems.
The jailbroken phone would crash almost every day, and was generally very buggy, but after upgrading to the redsn0w method a couple of days ago, the whole thing seems to be much quicker, more stable, and best of all, the battery seems to last a little longer.
If anyone is thinking about jailbreaking, make sure you read up on it first; you really don’t want to be losing any information, or having to restore to factory settings.
*I’m sure O2 aren’t that happy about people using apps like MiWi to get ‘free’ tethering, but considering I pay them a ridiculous amount of money each month for a tariff that includes ‘unlimited data’, I definitely won’t be paying them for the same thing twice. The fact they try to squeeze a little more out of customers in the way that they do is absolutely disgusting.