Now and have been since May, AT&T. Have a 4S on Verizon that I will be dropping ASAP. The service sucks at my house and drains the battery quicker than anything I have ever seen. Once I leave my house, my battery can last a few days.
I have 2 phones under 1 shared plan. The way I managed it was to make sure I was eligible for an upgrade every year in the month of October, hence, having each phone up for an upgrade alternating years. I'd hand down the 4S to my parents and get the 5. Next year, upgrade the 4S to the 6 and hand down the 5 and so on and so forth.
I think I might get the new iPhone off contract/unlocked at Apple and go by way of prepay. I don't need data so I wonder if I can get that to work with T-Mobile or Virgin Mobile? I'll have to look into it when it gets released.
I've got an upgrade just sitting in my VZW account. I'm on a Family SharePlan with myself, my Mom, my brother, and my grandparents. My grandfather is the account owner. As soon as my brother's contract ends in February (I used his upgrade in Feb. 2011 to pre-order my iPhone 4....), we're getting our own plan. Just my Mom, brother, and I. Unfortunately, Verizon is forcing that STUPID "ShareEverything" plan. They forgot to mention that not only do you share voice, data, and text messaging, but you also share the pain.