Short version: I am very happy with my Thunderbolt.
I have had a Thunderbolt since early this month. It is stock, unrooted, and came without any of the updates, and I have not had an update "push." I have had no random reboots. Only 4G issues have been the brief outage last week. GPS lock is usually fast, and I have one app I use that can clear and restart the aGPS if necessary (GPS Test). The build quality on mine is good.
4G speed is awesome (I had some interesting speed tests the other day - 6 down, 11 up, repeated once).
My only issue is battery life, but I spend most of my work day in a car, so the "Motorola Vehicle Power Adapter micro-USB Rapid Rate Charger"
Amazon.com: Motorola Vehicle Power Adapter micro-USB Rapid Rate Charger: Cell Phones & Accessories ($5.04 @ Amazon) is my friend, and I am probably going for the extended battery as soon as I can order the Seidio case to fit it. I'm also almost certainly getting one of the New Trent external batteries (IMP880 or IMP1100)
Amazon.com: New Trent iCruiser IMP1000 11000mAh External Battery Pack and Charger for iPad 2, ipad, iPhone 4 4G 3Gs 3G (AT&T and verizon) , iPod Touch (1G 2G 3G 4G), HTC Android EVO, Blackberry, Kindle DX, Samsung Galaxy S, Droid, Nintendo, Sony PSP for those times when AC or car DC is not convenient or available.