Yeeha!
Here's a thread I've looked at a few times, and have wanted to give my two cents. Now that someone else actually resurrected the thread, I will.
I've had a 12" in three episodes.
First, was a friend who went trekking in India, and left me to take care of his 12" for the 20 weeks he was away. That was in the midst of my darkest wintel-period (2001-2006) and while there was no way I could have afforded a 12" PB (I also had a lot of wintel-only software to run, and emulation is such a b**ch), but it rekindled the love, and created a strong desire to someday...
Second, was after I had already returned into the world of intel macs. I was offered a used 12" for a decent price and went for it. Sadly, one of the heat sink screw sockets was broken (I found out too late) and the machine suffered heat stroke. I unloaded it along with 7 other computers in an attempt to simplify my computing landscape (focus on intel macs running parallels and bootcamp)
Third was quite recently. I found a 12" in pristine condition on a local forum. I even spruced it up using an SSD (man! The utter silence of that machine was stunning - but it made the fan spin up even worse). As my mother-in-law suddenly needed a machine, I gave it away.
Why is the 12" perfect? Well, honestly it is not. It's quite thick and comparatively heavy. The heat it produces and it's battery life is not the greatest either. And for me, the 1,5 Ghz G4 (and ram limitation) is not snappy enough (I'm impatient).
But the machine is so utterly lovable. It's form factor and ergonomics is perfect, and I love the 4:3 screen. If someone could offer me a 12"PB chassis containing a C2D (even a low-end would do), with a SATA SSD, 4 gigs of ram, a modern trackpad and 4 hour battery life, I'd be in seventh heaven.
IMHO, the 13" MBP is the rig which comes closest* to the bliss-point which the 12" hit (especially in the early 00's) spot on.
RGDS,
* I have the feeling, that the 13" rMBP might actually be even better, but I haven't had the chance to get up-close and personal with one yet (fondling one at an apple retailer does not count)