My sister and brother-in-law both got D3's recently. It's a pretty nice device, my sis actually had me set hers up for her, so I got to play around with it quite a bit.
Frankly, after using it for a while, I came to the conclusion that I really do like the Thunderbolt better...and that's without even taking 4G into consideration.
I found the UI kinda...I don't know...dark and dreary? A lot of folks bash Sense, but I really like it, especially compared to the UI moto is using at the moment (not sure if it has a name or not). May have also just been the screen itself, but the UI was not helping. The overall "speed" of the device was pretty snappy, but it really didn't seem noticeably faster than my TBolt. My sis was happy with the keyboard, but frankly, i doubt I'd ever use it -- the larger form factor did make it better than the keyboard on the original droid however, I will admit that.
As far as I know right now, there is also no way to root the D3, or at least unlock the bootloader...which pretty much eliminates it from my list of own-able devices as well.
Again, just my take. I was actually rather impressed with it, and my sis is really happy with it (she had been stuck using an old BB 8330 for a few months waiting for her upgrade to become available) but in the end, I still liked the Thunderbolt much better.