I bought by Galaxy S3 from T-Mobile, in the US last year. So it is the SGH-T999 model. I've been very happy with it. I've stuck to running it as stock for a couple reasons. The biggest is that I just didn't want to hassle with learning how to change the underlying software. Secondly, because I wanted to keep everything cool in terms of the warranty and what not. I even jumped through all their hoops to get my unlock code from them.
But I ran into an issue the other week that finally used up all my good will towards t-mobile. I went to use the phone as a hot spot and couldn't. It wanted me to pay for their tethering plan - which doesn't exist where I live.
I didn't have time right then to do anything but this week-end I did a little reading and today I installed Cyanagenmod on it. So far everything seems to be going very well. And I actually have control over the device that I paid (in my mind anyway) a pretty hefty sum to own. None of the steps were all that difficult and my biggest holdup was it would not flash the new rom until the battery was over 85% charged - which I thought was odd, but it worked out.
I'm getting all my stuff back to the way I like it and enjoying being on a newer version of Android than the one that Samsung and T-mobile officially supply for this phone right now. Which is idiotic.
So - moving forward I don't plan to purchase phones from t-mobile any more. I'll probably stick with Nexus phones from Google and avoid all this headache. I wont get another new phone for a while. I bought a new, high end phone so that it would last a while. But when I do I wont make the mistake of getting stuck depending on mobile phone companies.