Running VMs for testing during development (ie: an IE6 VM, IE7 VM, etc) are the major driver for us.
Outside of that, my work machine has Chrome, Mail.app, Terminal and Textmate open generally so 2GB is fine without the VMs.
My home machine is the previous generation, top of the line iMac (3GHz, 2GB) and is often noticeably sluggish. It looks like iTunes is responsible for a bit under 200MB. I really don't know what the issue is though to be honest, but it feels like a 4GB upgrade would get it over the hump.
On my Ubuntu machine, I rarely use more than 1GB (apart from the disk cache, of course, which can fill all remaining memory. Note: windows only has an 8MB cache)
On my Kubuntu machine with 4GB, I rarely use more than 1.5GB of RAM, and that's counting the disk cache which will grow to fill all available space. Unless I run a VM, the system literally has no use for half my RAM, except for dual-channel access.
Can you qualify the "Windows only has an 8MB cache" bit?
Of course on Microsoft Windows it uses all available memory, quite effectively. If you have copious RAM you get far-better-than-SSD "read speeds" when reopening applications (which is a typical behavior, cycling between a small universe of applications), presuming your computing lifestyle is that your computer is never turned off...e.g. you put it to S3 sleep during off time.
