Three has Feel at Home and AYCE data. The former is seriously flawed, but good for a lot of people, the latter may or may not be required given we now have high allowances on other networks, like 16, 20, 25, 30 and even 50GB (the latter being silly money, mind).
After seeing what data abroad is like on Vodafone, I will pay £3 a day for unhindered 4G data over 'free' feel at home that's throttled to hell and back.
AYCE data is good, but my usage fits within the 16GB on EE and 20GB on Vodafone most of the time, so I am keeping AYCE data more for peace of mind, and perhaps the fear it might be axed or increased in price massively.
If I was honest, I don't really need it.
When EE formally launches VoLTE and 800, I assume the EE issued S7s (and other phones like the G5, M10, maybe the new Huawei P9) will all be good to go either immediately or after a SW update. Who knows with Three?