You can get a micro sim at your local TMo store. My store did not charge me.
I use an unlocked one on Edge speeds in the US. I bought the 64gb version from a US Apple store with Apple care for the warranty.
A huge benefit is that on Edge speeds the battery lasts about twice as long. I use about 5000 mins a month talk time and that extra battery life means a lot. When I travel I am almost always on wifi so no big deal in a hotel or cafe.
MMS do not work (at least for me). One of my best friends lives in Switzerland. We communicate for free with iMessage and facetime. Imessage allows transfer of pics and video with no limits. You can iMessage anyone with an updated iPhone that has iMessage.
Another benefit is when you go overseas you can buy a local prepaid sim and use the phone for cheap there.
I have, not once, wished I was using 3G or required faster Internet. The Edge speeds on TMo are fine for me, but may not be fine for you. To compare, the Edge speed browser on iPhone is about as fast as the 3G browser on BB 9780. iPhone might be faster to be honest.
Good luck and let me know if you have any questions.