Here's a few stuff about my experiences since I'm in the UK.
I arrived in 2010, signed to Voda, creepy service, quite quick web browsing but pathetic coverage.
I switched to T-Mo(at that time it was), which was amazing. Full coverage, little bigger latency and slower speeds, but calling and everything else was 10/10. I really loved them.
After my contract finished with T, I went to try a 1 month SIM with Three (it was just before the 5s launch). The sh*ttiest service ever! They have unlimited internet, which you cannot tether (only 2GB), they have 4G, but it's around 15mbit/s. They use O2's signal for 2G signal, which is pathetic solution to do. (worked in telecoms), they also offer you all these nice little things once you sign up, but after you wanna sign off... It takes time, I literally had 3 calls just to cancel my contract, and I even paid on top for some cancellation (I only signed up for 30days). Pathetic, and dodgy company. They also lend everyone a contract, litreally to anyone. My friend just came to the UK before the 5s launched, she got the phone on a 41£ contract (she lived in the UK for 2 months). Where some people like this, it shows the company's strategy.
When the 5s came out, I went to pick up one from store on the morning. (massive queue, 100 phones stocked in each store) I was out of luck. There wasn't any pre orders (BAD APPLE, BAD!).
I went to my local EE store, and asked if they got any 5s left. The stores were down that time, their system was dead so they couldn't do any UPGRADES, but they could do new contracts. I was happy, more than happy. I got the 5s on that morning, from a simple silly store, lol.
Since then, I'm on EE, love them! Their ****** policy on unlocking is terrible, I agree. You gotta wait 6months of you contract +25£ on top for ONLY IPHONES. Other phones can be unlocked for like 15£ I remember. (it's a bad joke)
At the 5th month, I filled a form, sent it out, and asked them for an unlock. They replied in one day, saying my phone is now unlocked and my next bill will be 25£ more. I guess thats a perfect move from them, really liked it as I was going on holiday next week.
Since then, I hadn't had a bad day with them. Once they shut off 4G for a day in my area, but nothing else really. 4G coverage everywhere, and guess what?
77 MBPS!!!
So no, no any other carrier, thank you. My mum's on Voda, got one signal at her house, my GF is on O2 and she says shes happy with it, (me too, as I often check what coverage she has, and it's more than satisfactory)
I dont want to make this seem as an advert as I hate these companies(inc EE) as well, they are expensive and ask for a ********* of money, but from this, EE is the best.
UI: about pre orders
I've pre ordered every iPhone since the 4, got every of them on launch day. Orders can be placed at 8 AM !!!! (as it was in the last 4 years) except the 5s.