I use an iMac 24" (3.0ghz) at work and a Mac Pro (2.66 nehalem) at home.
I am a software developer and I often use VM's which use up a lot of RAM and CPU power.
To me, the differences are as follows:
imac is ...
- prettier to look at (to me, this is important)
- built in video camera
- is completely silent
- i love the "plug and play"/"it just works" beauty and tight integration of it
mp is ...
- _noticeably_ faster in typical (browser/email/etc) usage, even with same amount of ram (honestly, i was not expecting this)
- you can load it up with more ram, easily
- you can swap out, install hd's, easily
- you can have a gamer-worthy video card
- you can plug in multiple monitors of your choice
- has workstation class build quality reminiscent of SGI and Sun workstations of the good old days.
i like both machines a lot. for most people, i would recommend the imac 24". but, a small number of people who really crave the expandability and power of the mp will find the much higher price of the mp worth it.