Been living with mine for a week now. It is, in every way, an improvement over the Thunderbolt. Well, every way but the keyboard. I had to install Swiftkey because I found the keyboard unusable. That's okay, Swiftkey is pretty awesome.
Now that I have the phone set up the way I want (rooted, wireless tether working, wifi notification and reminder gone, notification toggles themed in blue) I'm extremely happy with this phone. I usually prefer to stay more or less stock vs. a custom ROM so the bootloader doesn't really bother me.
I was using LauncherPro on the TB so I had to switch to Apex. I'm happy to say that it gives me all the features I enjoyed in LP in a clean, zippy interface.
Speaking of zippy, this phone is smoking fast. I don't know why anyone would feel the need to OC as the phone is fast enough as it is.
I love the hardware. It's incredibly thin and a lot of people say that weight makes it feel cheap but I don't get that sense at all. I like that the entire front is glass and the back is all one, easily replaceable piece. That makes it easier to keep it looking new. I'm also planning to order a replacement back so I can get rid of the VZW logo.
I'm not crazy about the side power. With the ability to turn the phone on with the home button I would prefer the power button be a little more out of the way. I do love the physical home button. I know lots of peeps hate it but turning the phone on with that button is far more comfortable than how I used to have to contort my hand to turn on my TB.
I loved my Thunderbolt when I first got it, but I definitely had more complaints right from the beginning. The SG3 is far better "out of the box" and with some continued tweaking and Jelly Bean I have every confidence we will be very happy together.