iSkin is very useful. Seriously. I don't want anything between my keys on the laptop, so I got that. The top row (esc, F1 - F12, eject button) doesn't fit very well, but the rest fits fine. But having said that, the protection it gives in day to day use is great.
I haven't found ScreenSaverZ to be very useful, but thats just me. I use it only while the lid is closed. That is coz I heard the keys leave marks on the screen. But I later learnt that this affects only the older PowerBooks (titanium I guess). But its v good to clean the screen and restore the shiny look of the Aluminium PB.
Hope that helps.