As an early christmas present my wife got me a new Samsung Galaxy S II from T-Mobile. This is by far the best gadget I've ever owned. My initial impression is WOW! It's very fast, much faster than my old myTouch or the LG that replaced the myTouch when it started over-heating.
The T-Mobile version of the Galaxy S II is a little different than the stock Galaxy S II. It sports a 1.5 GHZ Dual Core Processor (as opposed to a 1.2 GHZ Dual Core), a huge 4.5" Super AMOLED screen (as opposed to a 4.3" screen) with Gorilla Glass and a very slim, light case (it's lighter than my LG which is smaller) and a 1850mah battery (as opposed to 1650mah). The battery life so far is ok, not great it was useful for about 8 hours, which I guess is about the norm for android phones, at least the ones I've owned. It also sports an 8 MP camera on the back with an led flash and a 2 MP front facing camera.
The UI is great, it's using the Samsung TouchWiz 4.0 launcher. It's a very smooth UI and totally customizable. It comes with the Swype keyboard and a samsung keyboard which is very close to the Android Ice Cream Sandwich keyboard. It's running Android 2.3.5 but will hopefully be updated to Android 4.0 (Ice Cream Sandwich).
So hopefully I'll get a lot of use out of this phone and it'll last the whole two years that we re-signed the contract for. We've been very happy with T-Mobile and have no plans of switching carriers as you can't find better customer service from a cell phone company. But if you're in the market for a new phone, take a look at the Samsung Galaxy S2, you won't be disappointed.