... is awesome. I have the same keyboard at home and at work. It does everything that I want a keyboard to do. It looks fantastic. It feels fantastic. It types fantastic. And best of all, it does all of this without any drama.
I got my first Microsoft Natural Ergo 4000 after a gruesome compiler's project at school. My shoulders were hurting from the crappy chair and table combination, and my forearms were sore and numb from using the iBook keyboard.
I never used an ergo-split keyboard for an extended period of time before this. It's not that I hated them or anything, it's just I didn't really see a point to using one. After that painful 164 project, I figured that every little bit might help, so I gave the split keyboard a shot.
The keyboard came in a crummy cheap-o box with crummy software, and a crummy piece of plastic designed to be placed under the keyboard and get in your way. Take my advice and just toss all of this as soon as possible. Setup was super easy since it's just a USB keyboard. I opted away from the Microsoft Natural Ergonomic Desktop 7000 purely because wireless keyboards require too much maintenance. I don't care if battery life gets better and better. I know the one time I don't have pre-charged batteries will be the one time the batteries in the keyboard decides to die. I think the 7000 is a better deal overall if I didn't have a mouse already.
Overall impressions have been great. I never notice my keyboard these days. That's exactly how I'd like a keyboard to behave: working and unnoticeable. I had to unlearn a few bad touch-typing mistakes like 6, and 7. The other complaint I had was the lack of support for mapping the media buttons in OS X. I would've really loved to have the back/forward/scroll buttons work since they're near my thumb and index fingers. Thankfully, the back and forward buttons don't click anymore so it doesn't matter if OS X supports them or not.
I highly recommend the Microsoft Natural Ergo 4000 keyboard. In fact, I highly recommend all Microsoft keyboards and mice. They're always a good deal when it comes to comfort and features.