I had a 6700, skipped the 6800, and went to the Pro. I can't really compare the two, but I can tell you my experiences.
It feels SO much smaller in the hand (if a bit heavier). In portrait mode it's only a little bit taller (and almost the same thickness) as the free LG flip phone I got. People make fun of it being a "brick", but it utterly destroys phones like the Sidekick, and is pretty competitive with phone like the Envy 2 size-wise.
The screen is incredible. One of the best screens I've seen since the iPaq hx4700.
The keyboard, while physically smaller than my 6700, is awesome. The 5-row on the Pro is possibly the best keyboard I've ever used (including BB's and Treo's). That said, just like the 6800, one hand use is almost impossible.
Memory is incredible. I've yet to push this phone over 60% mem in use (even when using Skype, PockeTwit, and TomTom all at the same time).
I do miss having buttons, and the scroll wheel on the Mogul always seemed like a good idea.
Battery life has been MUCH better than the 6700, but I hear it's only "just" as good as the 6800 (though not bad considering the screen).
GPS gets a fix quickly, but I am suffering from the GPS lag bug (I always seem to show up on screen about two city blocks behind where I actually am).
Even with the Touch Flo interface, it's not a radical departure from the previous WinMo QWERTY sliders. If you liked the 6800, chances are you'll like this phone.