I bought the iphone 6 tmobile version online and picked it up in the apple store. I had my at&t sim card and told the apple store rep to let me test it and that I didn't want to activate service with tmobile. It worked right out of the box! I did this first because I was told previously by several customer service reps that I would be required to activate with tmobile in order to leave the store with the phone, and if this was the case, I would return it right then and there. I was not going to pay $706 for a phone that would require me to activate service with a company that I would not use.
NOTE: The Tmobile version comes fully unlocked if you buy it directly from Apple, either online or instore. If you buy it from Tmobile it WILL be locked to Tmobile. I am not sure about other retailers, but most likely they will be locked to Tmobile. I don't know what happens if you activate the phone with Tmobile first and then pop in an AT&T or other sim card, but it's better to just pop in your sim and don't bother activating with Tmobile unless you want to.
As for Verizon, I've been wanting to try out their service for awhile. I love AT&T, but I heard Verizon is better in some areas. Here is my experience:
I walked into a VZW store and told the store rep that I wanted to activate my phone with the $45 ALLSET plan that comes with unlimited talk and text and 500mb of data, they give you an extra 500mb free if you sign up for autopay. *I previously paid $45 with unlimited talk, text and 1gb with gophone which was great. Verizon lets you buy additional 500mb for only $5 etc. This is all to me much better than paying $100 a month on postpaid.
Now the first thing the store rep asked was if the phone was a Verizon phone, I said no, I bought it unlocked. He gave me the usual run around saying that the phone isn't compatible blah blah blah. I told him that this phone was unlocked and that it has BOTH cdma and gsm. He still told me that even if that's true, he wouldn't be able to activate it because the system won't let him. I told him, I'm aware of that which is why I wanted to activate the sim on a Verizon phone and then pop that sim into my phone. He was struck with curiosity more than anything, so he tried the sim from his phone on my phone and tested it, he was flabbergasted that it worked. So he agreed to activate the sim on another device and then was able to get it working on my phone.
OK, here's the catch. You will have a rather difficult time trying to get the phone activated with Verizon. I don't know if it's because I was setting up a new account or because I was transferring my number, or because it was prepaid, but I spent about an hour on the phone in the store trying to get my phone activated. It's quite possible that if you already have an active VZW account that it would be much easier since you could just pop in your activated sim and call it a day. The even bigger issue is that for some reason, I am not able to contact any customer service number from my phone (611, 228, etc) nor am I able to do it from a different phone when I am asked to input my number. I am always given the message that they are experiencing difficulties and hang up. I am also not able to sign up for my verizon, it says they are having difficulties as well.
So to sum up, the service is great, I get LTE, voice, sms. I don't get visual voicemail and I haven't gotten Volte to work. So I want to know how long it was for you to get it to work? Also, if I'm not able to make an account or call cust service, I have no way of loading my account every month. In order for me to put funds into my account, I had to call sales and have them transfer me directly to a prepaid cust service rep. I cannot be doing this every month, I can sign up for autopay, which I wanted to do, but I don't like the fact that Verizon could care less that you are paying for service if you are using a non Verizon device. The customer service just isn't there.
I will be testing out the service since I already paid for my first month, but if I keep having difficulties I will just cut my losses, and go back to gophone where I know that the phone is 100% compatible. Anyway, sorry for the long post, I just wanted to share my experience for anyone who is considering using the unlocked iphone 6 with VZW. If anyone manages to get all features working, please reply, Thanks!