I have one on my 12" iBook. Ditto on: you can close the computer with it on, and on the way to clean it (I also take it off and shake the dust off every once in a while).
I should point out a big downside...(bear in mind I'm still using mine). If you type FAST, based on touch, I find this thing to be kinda troublesome. I definitely do not type as accurately on my iBook keyboard when this thing is on vs. off. I think it makes the keypresses slower, so I transpose characters slightly more often, but the more annoying thing is I get a lot of doubled characters because of it. Right now, though, it's not so inconvenient that I stopped using it.
Probably the biggest reasons for that are my tendency to knock a shine into the keys without something like this, and how much dust shows up on it (which would be in my keyboard!)
Oh, one other thing...I think it might very slightly have a negative impact on the cooling airflow for the computer....