I got off T-Mobile about a week or two ago when my contract ran out, they are absolutely terrible.
When I first started with them, they were OK. Then it started getting worse, constant dropped connections - then this Orange merger made it even worse, constantly dropping to 2G. Low signal and finding a place to actually phone people or send a text without an error...
I was getting these speeds according to Speedtest (on 3G):
10am
DL: 345kbps
UL: 61kbps
Ping: 131ms
My friend in work was with 3 and was getting synchronous broadband speeds in excess of 1-2mbps at only 100ms ping!
I switched to them on "The One Plan" at a monthly renewal (to ensure I don't have to stay if they start fucking up)
I now get a HSDPA connection ALL the time (unless I'm in the basement in work), I get no problems phoning people or sending texts, and my connection speeds rocketed - as follows:
10am
DL: 4490kbps
UL: 1620kbps
Ping 83ms
Even when I phoned T-Mobile up a few weeks before I bought my contract with Three they were very rude as follows:
Me: Hi, I'd like to cancel my contract please.
TM: Yes, of course can I take your details?
*skip a bit*
TM: OK, we'll get that done for you. But first can I ask why you are cancelling?
Me: Yeah, I can tell you. When I first started with T-Mobile it was great, then you reduced data by half, joined up with Orange and the network got worse and worse. I have a friend in work who gets synchronous data in excess of 1-2mbps, with you I get 300kbps download and around 60kbps upload. Not to mention the constant disconnects and I pay £10 extra to you... for what?
TM: What phone do you have can I ask?
Me: HTC Desire.
TM: It could be a problem with your phone, Mr. Crawford.
Me: No, it's not a problem with my phone - I tried his SIM in my phone.
TM: But I'm telling you Mr. Crawford, it's not a problem with our network.
Me: No, you're not TELLING me anything. How dare you insult my intelligence, there is nothing wrong with my phone. It's your network. Now if you'd kindly stop talking to me and cancel my contract then I'll be happy.
TM: OK, Mr. Crawford - I hope you have a good day.
Me: I hope you don't *hangs up*.
They then sent a text asking for me to complete a survey which I rated horrible and left a comment why.
A few days later they tried to keep me on by offering £5 off my contract, you can guess where I told them to stick it.
TL;DR: T-Mobile are terrible, AVOID AT ALL COSTS.