This, older thread, is actually entitled
"AC Membership: Worth it?"
http://flyertalk.com/forum/showthrea...orth+it%3F
But you'll also find about a dozen, comprehensive, more recent threads with a cursory search.
Edit to add: a few of them:
"AC worth the $?"
http://flyertalk.com/forum/showthrea...ip+worth+it%3F
"Is Admirals club worth it?"
http://flyertalk.com/forum/showthrea...ip+worth+it%3F
"Worth it to join AC?"
http://flyertalk.com/forum/showthrea...ip+worth+it%3F
"Admiral's Club - Worth it?"
http://flyertalk.com/forum/showthrea...ip+worth+it%3F
"Admiral's Club (Value For Money?)"
http://flyertalk.com/forum/showthrea...ip+worth+it%3F
"Admirals Club worth it?"
http://flyertalk.com/forum/showthrea...ip+worth+it%3F
And those are only the ones that used "worth".
Many other useful, detailed and thoughtful discussions in the "lifetime AC" threads and the "QF club vs. AC club threads."