I can't speak to the other keyboards. Though I would point out that I have a smaller (imagine k400 without the trackpad and almost no extra space on the sides) keyboard for my PS3.
this keyboard is great. It is "light" and people keep saying it's "thin"... let me clarify, there is nothing "cheap" about this keyboard. It is not flimsy, but much like most other plastic consumer products, you can break it if you put your mind to it. I often hold it with my left hand on the side while typing with my right, if I'm standing.
There is a very accessible on-off toggle located on the forward edge.
I like the fact that the underside of the keyboard is white. It adds some style to it. Totally a matter of opinion.
The trackpad is very stable, it has a decent cursor speed, but it would have been great to be able to change the speed/sensitivity.
The width of this keyboard is my favorite feature. It is wide enough that it makes sense whether you're putting it on your lap or laying it on top of your dog. It is very balanced. I can't lay my chicklet PS3 keyboard on anything without it falling over, and it's too thin to place on my lap, unless I want to sit like a girl
Back lighting is completely unnecessary when they keyboard has barely any functions other than what is found on 99.9% of other keyboards out there. This isn't like your laptop, where practically every key has a secondary "Fn" use. Therefore, as long as you know how to type without looking, you'll never need back-lighting
With that said, this is not a great "remote" in my opinion. I have a windows media center remote that I use for WMC. This keyboard is for every other aspect of the HTPC.
Hope that helps
PS. I imagine a lot of that applies to the K700 as well. But I like the clean look of the K400, and I'm not sure that there would be an actual function to all those extra buttons on the 700. As I said, I don't believe I would ever use the keyboard as a main remote for viewing media.