Not having 4G on the Thunderbolt is like having a Ferrari without the keys
Coming from a DroidX which is paper thin and light, the TB was shockingly heavy the first time I held it but within a few days you get use to the weight and it actually feels like a solid built device.. it doesn't have that plastic feel.
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I'm one of those people constantly upgrading their phone and this is what I do:
I have a family plan with my parents added to the account. They use cheap little flip phones and never use their upgrades. So I now have 3 upgrades available to use... I keep my phone in mint condition and then sell it on eBay as slightly used but in mint condition which I can usually get around $300 for it. The $300 goes to the new upgrade so its basically like you're trading your phone in for free.
Another option... which also involves keeping the phone in mint condition is to sell the phone on eBay for $300 and then just buy the new phone at full price at $600-$700. Then it works out to you only paying $300-$400 which is close to what you would pay for an upgraded device.