I've had mine for a few days now, and it really does work well. It feels very natural to play games on the N900 w this unit in place.
I got the rubber buttons and am very happy with them. They do have a nice "grippy" feel to them, and I feel better knowing that they will most likely be easier on the keyboard.
The blue-on-black scheme also looks very cool, especially because I keep my N900's color scheme a dark bluish grey!
Has anybody noticed some slightly response latency when using certain keys? For instance, the SNES "B" button (bottommost key on the right side) is mapped to the Up arrow, which seems to respond more slowly than the other keys.
Wee! Got mine today. Works just awesome.
And it's a bit too narrow like everyone has already pointed, but I don't think it's a problem. It sticks on the keyboard nicely and doesn't come off that easily.