As you can see from the list yes it is rare and It will be useless to me as I'm also not in the US so I will end up roaming on AT&T instead if I had a quadband.

Yes T-mobile USA use it but it is more a users initial network choice than a comparable spec. But if you're interested there is a T-mobile pentaband version of the Galaxy S2 with that band being released (October 12) called the Samsung "Hercules" specifically for T-Mobile.

Not sure what you are getting at but HSPA+ does equate to an actual speed difference. Sure you don't get the theoretical maximum but the higher theoretical maximum results in an actual real world speed gain over HSPA.

Regardless of where you live I think the S2 will outperform when it comes to actual cellular capability. For example if you were getting an S2 in the US you will benefit from WiMax and HSPA+ and can still roam outside the US. If you were outside the US you will still benefit from HSPA+ and can still roam just as well in the US.