The new 21UX (DTK-2100) is AWESOME!
Not only is it a nice looking device, but I could swear it sports a new LCD panel too (something that critics have panned about the new model as lacking... 5 year old technology, etc. I don't believe it). The firmware / OSD is definitely different, but the screen also seems a lot sharper with much better contrast, and without the washout issues that all the older 21UXs I've used before have exhibited. The new body has a texture similar to the 12wx, kind of a pebbled matte black. The new buttons are cool too, and them putting the touchstrips on the back like the 20wsx is also killer (no more hitting them with the side of your hand as you brush over them). The touch strip function toggle button is a great idea too. Overall, it's definitely a step up. I'm mixed about the lack of OLED labels for the buttons. While it would be "cool" for them to be there, I don't think it's as necessary as it is on the Intuos4, as you can tell what button you have your finger on without looking at it, since they're all angled in a unique way relative to each other. Of course, there's also the new digitizer. Can I tell it's got twice the pressure sensitivity? Well, for most of what I use it for (Photoshop, Illustrator, Flash, Maya, ZBrush), I can't, but I can tell you that it does take a lot less pressure to get pixels flowing from the tip. The new stylus and stylus holder (with nib storage) are nice too!
Of all the Cintiqs I've played with over the years (18sx, 21ux, 20wsx, 12wx), this new 21UX definitely has the best first impression. I used a 21UX at work for years (both the original and the upgraded versions), and I never was interested in having one at home, mainly because of the screen contrast and color inaccuracy problems (the fact that the ones at work were on 24-7 for 4+ years probably didn't help them at all). All in all, in my opinion, this version is a huge improvement. I like it.