Here's my impressions of the Das Keyboard Professional after using it for 3 days. I've been using Model Ms for several years so that's what I'll be comparing it to.
I've noticed 2 main differences:
1. The feel, The Das Keyboard has a slightly lighter feel. I find the difference is more noticeable when I release the key, rather than when I press it. The Das Keyboard feels a bit mushy when it reaches the click point on the way back up. On day 1 I noticed the T key was stuck at that point, though it only happened once. Also the Das Keyboard feels a little more grating/uneven when you press a key, compared to the Model M, which is smoother.
2. The sound, the IBM just clicks, the Das Keyboard has a bit more of a white noise sound to it's clicks. I saw it described as a "snick" on Ars. Also I notice the sound of the keys bottoming out a lot more when typing on the Das Keyboard.
I mainly program, and play games, with a bit of writing on the side. I've done more gaming than anything with the new keyboard so far, so this is still a tentative opinion. But so far I'd say I prefer the Model M for writing/programming, and the Das Keyboard for gaming, mainly due to the n-key rollover, though the lighter feel is nicer for gaming as well.
I bought the Das Keyboard hoping for a Model M with N key rollover, "windows" keys, and slightly less noise. It's close, but not quite 100%. I'd guess the main issues causing me to not like the feel of the Das Keyboard as much, are the slight grittiness, and the mushier key return of the cherry key switches. I could imagine both improving a bit with some wear.
Does anyone know of another Keyboard that would be a closer match to a Model M with N key rollover and "windows" keys? I know I can get "windows" keys from unicomp, but they don't do n-key rollover. Other possibilities I'm aware of would be a Filco or Avant Prime, both available now at around $150, and possibly a Deck Legend or steelseries keyboard with brown/blue cherries in the future.
Has anyone here replaced a Model M with an Avant or a Filco and been happy with the change?
A final small difference is the Das Keyboard Professional's complete lack of PS/2 compatibility. I use a PS/2 KVM switch and my Das Keyboard doesn't work with it. Though at some point in the near future I'll be switching to a new KVM switch with USB and DVI so it won't a long term problem for me.