Apple computers hold their value for years to come and my personal experiences can attest to this.
Right now I'm using a second hand BlackBook; the black MacBook released a few years ago. This model is from 2008.
Upgraded ram to 4 gigs, upgraded HDD to 5600rpm. 2.2ghz. I went into an Apple store for kicks with the machine and ran it side by side with the iMacs and MBPs (all with iCore processors) and loading web pages was virtually the same, opening programs like iTunes and browsers maybe trailed by a second at the most.
Of course, all of this isn't really CPU intensive but it's what I do on a day to day basis, 90 percent of the time.
You'll also see people talk about using their Mac for years without slow down or lag. It's great having a computer that doesn't break, slow down, or get "old" on you after a few years. You just keep using it until you need something better.
Personally I wanted to get the MBP after my friend got his (2010) but right now the only advantages he has over mine (form me anyway) is the backlit keyboard (which only comes in handy when my friend turns all the lights off at his house) and having a longer battery life (which was remedied after getting a spare battery off of ebay).
Oh, and the multitouch trackpad but I love keyboard commands so that isn't much of a problem either.