My (prototype) N900 sometimes just doesn't react to normal finger presses, but works once I use the nail. However, strangely this seems to be mostly limited to the desktop and the program selector; I guess it is a implementation issue there. I never encountered it inside Apps.
OTOH my prototype can get sluggish when busy with phonecalls. Yesterday, I was even unable to finish a phonecall, because the GUI hung (first time in a month). Had to remove the battery...

However, I'm really abusing the device in terms of devel-software, data and teststuff installed, so really trying to get to the limits. I have 2500 songs on it, a few hours of video and over 1000 pictures.

This device is really a powermachine: You can e.g. use your browser to get a torrent, then get the torrent DivX and watch them with the inbuilt Media-Player...