[Thunderbolt Q&A, Help & Troubleshooting] RunNgun42:
Let me offer my feelings, because I have just completed the exact same journey you are embarking on, meaning I had a Sensation on Tmo, and just witched to VZW for the Thunderbolt. Here are my impressions:

HTC Sensation 4G:

I used to be on my company's corporate plan with blackberrys but recently accepted a new job, which allowed me to get whatever phone/carrier I wanted. I had used Tmobile years ago and did the whole HTC HD2 custom ROM thing and had a really good time with it. The sensation always claimed to be on 4g mode and I got relatively good speeds. I could always stream music/videos without any delay, and Speedtest would report about 3mbps on average, with a peak of 8 on a couple of occasions. Coverage was excellent & no dropped calls.

The phone itself is a work of art. It's very light, had outstanding battery life under heavy usage, and Sense 3.0 is beautiful. The tweaked functionality of the camera app among other things was nice. My favorite feature of course being free built in hotspot/tethering. However something was missing.

I really missed the screensize of the HD2. While both are 4.3" diagonal, the S4G has a higher aspect ratio, so the phone is actually skinnier. I didnt like this. I really liked how readable text was on my HD2, and the proximity of icons/buttons when texting were. All in all, the HD2 was simply the perfect size for me.

Then I went to a VZW store and made then unbox a thunderbolt for me so I could try it out without those security wires attached to it. I immediately fell in love and knew within seconds that this was the phone for me. I ran out the buyers remorse period on tmobile just to make sure, and finally made the switch on Monday! I cant tell you how pleased I am. The phone just feels right in my hands.

Every time I used the S4G I was constantly judging it for its screensize. This was a huge bummer, because I could never enjoy the phone. Every time I picked it up all I could think about was how much more smushed and how much scrolling I needed to do to get the same screen real estate visible on the Thunderbolt.

Thoughts on the Thunderbolt:

People arent lying, the battery life is borderline abysmal, but this is actually fine with me since I work behind a desk. I am actually charging it via USB as we speak. I'll probably also plug it into my car charger so it's boosted by the time I get home. I seem to get about 4-6 hours of heavy usage out of it (I was recording myself giving seminars this morning, then surfing all through lunch using the kickstand). I dont mind the battery life since I can easily keep it juiced, but I would NEVER recommend this phone for anyone on the go.

The only thing I actually dont like is how bulky the phone is. Even my HD2 was slimmer/lighter. Maybe its the LTE modem, or maybe it's the kickstand, but the phone feels fat in my hand. In fact, I've basically traded one nuisance for another, nagging on the S4G due to size, and nagging on the TB due to weight. Time will tell if I begin to regret my decision, but I think the other benefits will make up for it.

On TMO, within my first week on the S4G I had already used 1gb. Granted I was playing around with it a bit more than usual, but still. I think we're all gonna hit 2GB a lot sooner than we realize, and the carriers on banking on this (hence the reason they made them). Having 100% unlimited grandfathered 4G with the TB was probably my second biggest selling point. I dont like using a phone and constantly checking the MyAccount app, or 3g watchdog to determine how close I am to running over. It kinda sours the whole experience. With the TB I can just kick back relax and know I'll never be bothered with it.

And speaking of bulk/kickstand issues, I just want to say how AWESOME this kickstand is. I am constantly using it (even though it's only been 3 days lol). I love being able to stand it up and flick through web pages with 1 finger while I eat a sandwich or pasta or something at work. I'm really shocked they arent doing this will all their phones now.

Oh yeah, one other thing, performance. Believe it or not, but the TB actually feels snappier to me running Froyo than the S4G. Dual core or not, the S4G's framerate when switching homescreens always had a little choppiness to it. This really blows and feels like you're using a bogged down phone. The TB always has wonderfully smooth transitions. Obviously this is due to lack of 3d fx, but I'd rather have a smooth simply interface than a clunky advanced one. The only feature of Sense 3.0 I actually used was the lockscreen widget. It's pretty hand to be able to open emails/sms messages from here, but it really only saves you from 1 extra step when you use a standard lockscreen. All the other upgrades apps were pretty useless. Yeah the new weather widget is awesome and I would just stare and stare at it for minutes at a time. That was on my first day. On the 2nd day I never even noticed it. I even used the weather as my wallpaper, and still I didnt notice it.

P.S. I was really nervous about this whole ATT thing too. ATT has given some blanket statement that existing 4g users on Tmo's network will be "given an opportunity to upgrade" or something along those lines. Basically you WILL have to get a new phone eventually. LTE is NOT what Tmo is using, and ATT wants that spectrum baaaaad. Believe me, once it's theirs they are going to use it, and completely redeploy it for real 4G as they can. If you're LUCKY, they MIGHT do it in a series of phaseouts, letting users of phones like the S4G exhaust their contracts before each little piece of spectrum is gobbled up and transformed into LTE space. And thats assuming they're going to play nice, and since when has ATT ever played nice? ATT is always the first carrier to impose some asinine rules or restrictions on their services. They are the last company I would trust to be fair and not just force me to renew my contract in 13 months or be dumped to 2g speeds forever.

Hope that helps!