While I enjoy and am keeping my TB until it doesn't work anymore, there are 3 things that would make me think about switching phones:
***Please refrain from arguing any of these points because we all know them to be true and no matter what resolution you suggest, they've all been tried and have little affect on the outcome***
1. Battery life
Fine on 3G, horrible on 4G. It's supposed to be a 4G phone, need I say more?
2. GPS
I'm in NYC and can never get a quick lock on GPS when I use navigation. Plus, no matter where I am in the country, the navigation takes 20 minutes plus to lock onto GPS.
3. 4GLTE connectivity
I'm in NYC with complete 4GLTE coverage and when I turn 4G on, I'd say 50% of the time when I pick my phone up to use it, I don't have a data connection. The battery also drains ridiculously fast with 4G on (100% to 25% in 5 hours with barely any use... seriously?). I use 3G almost all the time because of this.
Like I said above, I do enjoy the TB and will be keeping it for a while because the 3 things above are manageable. But if a phone comes out that works the way it's intended to work on 4G, I'd give it serious thought (but probably stay with the TB for a long time after it's released)