I have a 10Gb ipod, which is more than enough - I've got a fairly sizable music collection, but 10 gigs is enough for all the stuff that I listen to regularly. If you could fill a 40Gb ipod then i'd bet that you'd almost never listen to most of it.
The ipod itself is great, and it's simplicity itself with a Mac. I remember the PC as being a bit more of a pain to use, but I didn't really try that hard.
One gotcha if you're using old hardware - the firewire port on my old Sony Vaio didn't work with the ipod, so something has changed at some point.