Here is some feedback on the MacMini from a MacCast listener.
…More than anything, the unit is the perfect size (not much bigger than an external hard drive). In addition, it is very quiet, though the fan will run at full speed under long periods of very high CPU usage. After the CPU usage drops, the fan quickly silences.
Coming from a 667MHz PowerBook G4, the Mac mini is a respectable upgrade and would make a very capable auxiliary, or even primary, computer for most users.
Below is a picture of the Mac mini next to my external hard drive.