I upgraded my wife's iPhone to iOS 8 (GM seed) last night. We switched to T-Mo about 2 months ago when VZW's announced "network optimization" policy was targeted at Grandfathered Unlimited customers like my 5 lines were.

For the most part T-Mo has been great. We are in the STL suburbs and their coverage certainly is not as robust as Big Red, but when they do have good coverage, my LTE speed tests on my Note 3 indicate it is faster, which really shocked me. Plus, the monthly cost for the service is lower than it was with VZW.

However, the coverage and call quality experience for my wife has been mixed while she was on iOS 7 with her new 5s. Their signal is pretty lackluster in our subdivision, and we had to get a cell booster unit from them (free of charge). That did help some indoors, but it is the older model, and applies only to voice and HSPA, and not LTE.

WiFi calling for me on the Note 3 has been fantastic. The call quality is way better than typical, and almost HD-like. I also know that T-Mo supports VoLTE under certain circumstances, also providing superior performance when off of WiFi. But, remember this applies not only to T-Mo at this time, but also to devices they sell with their firmware image. You cannot, for example, bring an AT&T GSM Android device to T-Mo and expect to get WiFi calling to work.

As of right now, on iOS, Wifi calling is only supported on the 5s and 6 running iOS 8 or later, and on T-Mobile. I don't think the other carriers have worked out the support yet with Apple, but I would be surprised if wasn't.