The K350 is a great keyboard for the price.
I have a Microsoft Ergonomic 4000 too, so comparing the two was an interesting exercise in competing design philosophies. Unlike the MS 4000, the Comfort Wave keyboard doesn't feature a split key layout. You're essentially given a conventional keyboard with a padded wrist rest (super comfy), and a slight semi-circular upward tilt to avoid excessive pronation of the wrists. The ‘wave’ in the keyboard's name comes from the differing height levels of the keys. Each of your fingers differs in length, and Logitech's design factors this in by having the ‘K’ lower than the ‘J’ due to your middle-finger being longer than your index. For the most part, it's a subtle alteration and works quite well.
You have a variety of media keys (play, pause, mute, etc.), several programmable function keys, an illuminated battery indicator, and a selection of Windows-specific keys to open things like the Windows Media Centre. I don't run Windows so some of these were redundant, but all of the generic stuff functioned quite happily under Linux.
The Comfort Wave's form factor isn't as hungry for desk real estate as the MS 4000, and the keyboard is incredibly light. It takes two double-A batteries and there's an on/off switch on the underside. You can raise the rear of the keyboard with two height-adjustable feet which give you the option of either a 4 or 8 degree positive tilt. Unlike the MS 4000, there's no way to adjust the keyboard for a negative tilt (the rear facing down), so if that's something you're looking for, bad luck. (You could always stuff a rolled up tea towel underneath the wrist rest or something, but that'd be a bit of a bodge job.)
In terms of build quality, the Comfort Wave is a mixed bag. The wrist rest is great with some high-quality padding and the plastic surround is assembled well. The keys could be better. They seem a little cheap, and while there's a nice tactile bump whenever they're depressed, the MS 4000's keys have the edge here. They're not bad… But they definitely reinforce the notion of ‘you get what you pay for’. They're also quite ‘clacky’, so if you're after something quiet, this is not the droid you're looking for. Given what I paid for it, I'm reasonably happy. It's a comfortable keyboard to use, and typing is a pleasant experience.
The Logitech M510 laser mouse that comes bundled with the keyboard is great. It has enough weight to make it feel substantial without giving the impression you're pushing a brick around your desk. It's comfortable in the hand, the buttons are all well-positioned and the switches are of good quality. You can also side-scroll by pushing the mouse wheel left and right, which is a nice feature. It's an impressive piece of kit. Like the keyboard, it takes two double-A batteries and has an on/off switch on the underside.
The MK550 bundle is a solid investment for those looking for comfort without having to take out a second mortgage on their home. Recommended.