Projects where the Pi is useful are things where you have data to gather ( slow data ) and you want to graphically display it. It is a small sub set of all possible projects.
Something like a home security web based system sending web cam data over the net.
In my wand project
http://www.thebox.myzen.co.uk/Raspberry/Magic_Wand.html
I minimised the disruptive effects of Linux by using a hardware time to generate the delays. The program spends most of it's time in these and so is not affected unless the time is stolen just close to the very end.
This picture is one of the "out takes" from that project and illustrates the point. In actual practice you don't notice this very much but it can be captured in photos.
