A mac is always a good investment, they last for ever. When I get one I always get the top of the line fully optimized, they last 5 years easy. I still have a 12" Powerbook from 2004 up and running. 17" Mac book pro from 2008, A Mac Pro from 2008 and this new one 15" MBP from last year.
I agree. I have a lot of Macs that I have been collecting since the 90's. While they all don't last forever, most over them do.
The ones that no longer work are the Power Mac G5, and the iMac G5.
I have been really impressed with the 2006 Mac Pro 1,1. I had to do the boot loader hack to upgrade it to Mac OS 10.9, but it runs great.
My favorite overall would probably be the iMac G4.