here is my take on it:
ive had 4 machines now
1.83 CD white macbook (still in my possession)
2.2 C2D white MB (gone)
2.16 MBP
and now a 2.4 MBP
the 1.83, bless its little cpu heart, is cracking all over like a plumbers butt! granted it was the first macbook i pulled apart, so it may be myself that caused it, but i doubt it. the left side has two major seems starting. One at a screw (left side closest to user), and on in front left corner. This machine wouldve probably loved applecare. the battery was replaced 2x (in the first year though), and nothing more
the 2.2 MB, which i ebayed, was junk. it was wearing like crazy after only 4 months of light palm use. the material felt cheaper.
the 2.16 MBP, was actually replaced within a day. IT had 40 dead pixels on my return home, which i was very mad about. again, manufacturing defect.
my current purchase, the 2.4 Penny MBP, is cake. nothing wrong with it at all.
i always wait till the end of the first year and decide 2 things:
1. am i going to keep the machine (if not, i sell it 10 months into ownership, so new user can buy applecare if they want)
2. have i had issues with it? build quality holding up? etc. if any of these are negative responses, i probably look into apple care, or sell the machine.
to this day, i have not kept a machine more than 10months, with the exception of selling the 1.83 to my girlfriend, who by the way loves the little guy.
applecare? if you have the money, and plan to keep the machine, do it. If you like to upgrade like me, its better spent on a new machine.
edit: i think we all agree that cpu power has reached a point where Most of us dont need any more. the few that do, are those cutting edge guys (like me). i dont need more, but itd be nice
if the machine serves your purpose in 1 yr's time, chances are it will in 2.