Now that I'm back from my trip I thought I'd add my own answer with details of the experience. All of the following is using an iPhone 4S, unlocked and using T-Mobile's Simple Choice plan (originally an AT&T locked phone).
On landing at Heathrow I turned off airplane mode and my phone immediately connected to EE. I've seen reports that this could take several minutes, but I was online before the plane reached the gate (literally, exactly the same as when flying domestically in the USA). T-Mobile sent me a couple of messages:
Data was functional but, as noted, somewhat slow. After a couple of days T-Mobile tried to upsell me on a fast data pass, which I decided against.
Partly I just didn't need the faster data, and partly I had signed up for The Cloud, which offers free Wifi access at numerous UK locations. Often when I'd pull out my phone I'd discover that I was actually using Wifi data instead of phone network data.
I stayed the UK, mostly in London with side trips to Cardiff and Milton Keynes. Most of the time I was on EE, though I also saw O2, Vodafone, and Three at times. As advertised, my phone just worked. Calls and texts to my US phone number showed up as fast as usual, and calls to the US went just fine. I did not make any calls to UK phone numbers-- I expect I would have had to include the UK country code, but otherwise I'd expect them to just work.
Usually that was fine, with a couple of caveats:
- EE's coverage was not always as good as I would have hoped. Usually it claimed "3G" and worked fine:
- Nevertheless, Edge was common and GPRS showed up at times.
- O2 never worked, not at all. If I saw O2 at the top of the screen I just gave up.
- In an attempt to reduce problems with O2 I tried turning off automatic carrier selection (on an iPhone, this is Settings --> Carrier --> Automatic) so that I'd only use EE. That stopped O2 from showing up but also meant I got dead spots. Dead spots on trains in relatively remote areas isn't a surprise, but I also got them in central London a couple of times.
My verdict: Taking Simple Choice to the UK works as advertised. In some cases local carrier coverage may be less than you'd hope, but that would apply to any phone and plan.
Update: The 20 cents/minute charge for phone calls may not show up until the next month's bill after traveling. The charge applies, but it may be delayed. VOIP options like Skype or FaceTime would of course count only as data, assuming the connection is good enough to use them.
Update #2: On a later trip that covered Iceland and the Netherlands, I had similar results though without anything like the O2 problems I had in the UK. On that trip I ended up buying a T-Mobile data pass to get faster data, but the basic service worked as advertised at all times.