I've owned two of the original K400 keyboards. The first one had the touch-pad die after 2 years, the second didn't survive a short drop onto a carpeted floor. Hopefully, this new model has a bit better quality control.
The Plus appears almost identical to the original K400. The volume keys are easier to reach above the touchpad, the mouse buttons are a bit smaller and more difficult to use. The keys have more travel, better feedback and feel more like a regular keyboard.
The keyboard layout is excellent for a media keyboard. The alphabet keys are full sized, and touch typing is easy. Many of the minor keys (Ctrl, Alt, etc) are half sized. The Plus has a slightly better layout than the original, with full sized Shift and Enter keys.
The touch-pad is an excellent multi-touch that supports two-finger scrolling, panning and tap-to-left-click (assuming your OS has these features, of course). The keyboard is Android friendly if you have a USB adapter on your Android (however, not all versions of Android have the K400 driver). The volume Up and Mute keys work in Mint Linux, but not the volume Down key.