First, I was nervous about getting a dud, but seems good out of the box. My D1 had gotten SO slow and laggy, and kept getting low on memory even though I couldn't find any more apps that would let me move them to the SD card...it was really on its last legs.
The D3 is a breath of fresh air! I love so many small things about it, starting with the power button that bugs some...I always found it toggled too easily on the D1, so I like the change. I love that the speaker is not a recessed dust collector as on the D1. The new keyboard is terrific, and I like having Swype native rather than the beta app add-on that I kept having to re-install.
I like the quick-contact icons on the home screen a lot, as well as how snappy screen-scrolling and app drawer are.
Call quality has been good, and I was always one of the cheek-dialers with the D1 and I haven't had that happen once yet (but it's only been a day). Battery life isn't bugging me yet, but I'm expecting it to be lame for the first few cycles. I did turn off the global radio and have it using only CDMA.
The screen looks really good to me. The only minor grumble I have is that my hands aren't huge, and it's a bit awkward to grasp the larger phone now, and it feels notably heavier (probably mostly in my head due to the awkward grabbing size). I'm sure I'll adjust, though, because I am LOVING this phone! I almost held out for the Bionic, but my D1 was dying too quickly and I really wanted the keyboard (and doubt I need 4G).
If you liked your D1, you will NOT be disappointed with the D3, I think. It's all that and a bag of chips.