I'm a big believer in having the best tools possible for the job. I've gone through at least 3 Microsoft Ergonomic Keyboard 4000's, and used to think those were the best keyboards ever (as did Jeff Atwood).
Well, there is whole world of keyboards that are even better, namely: buckling spring keyboards (e.g. the IBM Model M), Cherry MX switch keyboards and Topre switch keyboards.
I got this one for my place - Cherry MX brown switches, which are less noisy (though definitely they could be even more silent):
The major difference between a mechanical keyboard and a regular one is in the feel of the keys. Mechanical keyboards have metallic springs, while cheaper keyboards just have some flexible plastic. After a short while typing on the MS Ergonomic keyboard started feeling like typing on a mushy piece of plastic compared to the mechanical keyboard. I do kind of miss the more ergonomic split layout, but typing is just a lot nicer overall on a mechanical keyboard.
I ended up buying two keyboards, both with Cherry MX switches (~110 USD from here). I didn't get a buckling spring keyboard, because they are more noisy and there aren't any space-saving tenkeyless models available. The third alternative - keyboards with Topre switches - are more expensive (~300+ USD) and have been described as less tactile. I'll probably get a Topre keyboard eventually anyway, just so I can try it out.
Here is the new keyboard I use at work, which has Cherry MX blue switches (a bit more noisy than my other keyboard, but definitely one I prefer):
Best purchase in a long time!