Been using the MX Board 6.0 for about two years now. My impressions are in line with everything VSG wrote. The rubber from the wrist rest started peeling off just a couple of days ago - nothing that a bit of glue couldn't fix. Also, I'd love to have an ability to force some keys (Windows, Caps Lock, Num) to be red all the time, instead of switching to blue to indicate their on/off state. Other than that, I love this keyboard and enjoy typing and gaming on it. I'm dishing out something in the lines of 1.5-2 million characters per year (well, that's a rough estimate) - the keys still feel like they did when I first got it. All of them.
I'm actually currently growing a bit bored of this keyboard, but having a hard time replacing it - almost none of them sport an equally impressive all-metal chassis and look nearly as nice. The only one that comes close (from the ones that I tried out, at least) is the Das Keyboard 4. No wrist rest on that one though, so that's an instant no-go for me.