DX to S3 for me also. I loved my DX (got it on launch day) and that phone really just kept trudging along. These past few months have been pretty rough - the DX slowed down a lot and random reboots when using navigation got annoying - but I held off from getting the Galaxy Nexus because the hardware just didn't impress me and I wanted to see what Samsung and Moto would put in their flagship phones. Unfortunately, Moto's Razr screens were ugly and they waited too long to announce/release the Razr HD so I had to jump on the S3 before I lost unlimited data.
My S3 came on Friday and it's been great so far. It got up and running quickly and wow is it fast compared to the DX. When you get the phone, just open the camera app and snap a picture. I'm not a big camera user but on my DX this task would have taken about 20-30 seconds. On the S3, the camera is ready to go and can snap off a shot almost instantly.
When I'm not messing around with it and turning the brightness all the way up, the battery life really seems to hold up. On my DX these past few months, the screen would be off and it'd be sitting on my desk and suddenly I'd only have 30% battery left by the early afternoon. We'll see how the S3 holds up as this is my first workday with it.
I'm still getting used to the button layout and menus on the S3 but I'm liking it so far. Only real annoyances are the brightness, which is set too low when on automatic, and Verizon messing with touchwiz so wifi is no longer a toggle, it's an annoying persistent notification that can't be swiped away. Both very easy fixes - one you can do by yourself in the settings menu, and the other Verizon/Samsung could fix in a minor update (but I'm not optimistic that we'll see this).