When I thought about it, my reasoning boiled down to five main points:
It looks good: Thank you, Jonathan Ive, for making the iMac sexier as it gets older. The iMac has gone from hideous bright translucent color plastic to the first flatscreen G4 models, then to the white plastic all-in-ones to the current aluminum and glass models. The displays have become better over the years, with the 27" LED-backlit screen being the latest example of Apple's display prowess (initial manufacturing glitches notwithstanding). The thought of having that gorgeous aluminum slab sitting on my desktop has actually motivated me to do something I haven't done in years -- clean my desk!
Nice to have everything in one box: One thing I loved about my original 512K Mac was that everything was in one box. You picked it up by the built-in handle, and you could stack the chunky keyboard and mouse on top for an easy two-handed carry. When Apple got into the weird years in the early 90s, we all had a box -- the computer -- and a display sitting on top or to the side of it. The iMac brought the concept of all-in-one computers back to desktop machines. When I have to move an iMac, I don't have to worry about unplugging a monitor and moving it separately. I just unplug any USB and Firewire cables I have jammed into the ports on the back of the iMac and pick it up. In that respect, the iMac is very much the desktop equivalent of a laptop.
I want to get down to two devices -- iMac and iPad: I'm serious about this, and I'm moving in this direction. I don't feel that I need a laptop computer for my mobile work anymore. Instead, I will use the iMac for most of my writing, video, and photography work, and opt to use the iPad when I'm not in my office. If I need to use some of my "real business apps" when I'm bound to the iPad, I just use iTeleport or LogMeIn Free (still trying to decide which one I like the best) to control my existing iMac. Sure, it'll be interesting to crunch that 27" display onto the 9.7" iPad display, but I'm used to scrolling around my 20" iMac on the iPad so there shouldn't be too much of a learning curve.
Faster and cheaper than a Mac Pro or MacBook Pro: The 27" Quad-Core 2.8 GHz Intel i7 iMac that I have my eye on will dent my wallet to the tune of about $2,568, including AppleCare, 1 TB of storage, and 8 GB of RAM. A Mac Pro with a single 2.66 GHz Quad-Core Intel Xeon processor, AppleCare, 1 TB of storage, 8 GB of RAM, and a 24" Cinema Display would cost me about $4,087. If I wanted to go with a 17" MacBook Pro as my primary machine (I don't, as I want more screen space than I currently have), I could get the new 2.66 GHz i7 model and add AppleCare, get the 500 GB drive (1 TB isn't available), and install 8 GB of RAM. The total? $3,248.
Lots of real estate on that screen: That's my biggest reason for wanting the 27". Especially when I'm setting up my weekly TUAW TV Live show, I'm very constrained by the 20" screen I have right now. Erica Sadun thought I should go with a Mac mini and then buy an inexpensive 27" monitor. While that would be less expensive than the iMac solution, it doesn't give me a top-of-the-line CPU or that all-in-one design that I like so much. When I buy desktop machines, I expect them to last me at least four years, and by 2014 I don't think I'll be happy with the 2.66 GHz Intel Core 2 Duo processor that's currently the fastest in the mini.
While it has been overshadowed by its flashier portable cousins in the MacBook family and isn't the computer of choice for professional designers (who all seem to flock to the Mac Pro), the iMac still has a lot of life left in it.
Before I get a lot of comments telling me that I'm insane to want an iMac, just remember that every person has their own set of personal requirements. What's right for me probably isn't going to work for you, and vice-versa. All I know is that I'm going to enjoy all that screen space and speed in a few weeks.