I have a late '08 unibody. Last year I pulled the optical drive, installed an OWC 256GB SSD in that slot, and bumped the RAM up to 8GB. I left the original HDD in the original bay w/ the rubber bumpers. Note that you will only achieve SATA III speeds w/ the 2008 bus, even if you install a faster SATA drive.
OS was installed on the SSD along with all applications, and the Music, Pictures, and Movies directories were symlinked to the platter drive. I also moved the virtual machines over to the platter drive to keep the SSD at around 50% usage.
It ran Mavericks perfectly, and as of this weekend is also running Yosemite perfectly. Don't get me wrong ... it's not going to be in the same realm as the new rMBP machines, but considering its age I'm honestly surprised at how well it does. It's no problem to have a Linux VM up, a Windows 7 VM up, Photoshop, Xojo, and various other programs simultaneously, and it handles it quite acceptably. Boot times are in the 12 - 15 second range, and most all applications launch within a few bounces at most.
All that said, I did just order a new 2014 rMBP 15". This was my year to upgrade, as 6 years is a bit long in the tooth for my daily use computer. But I won't be selling or giving the 2008 model away ... it runs too well to chuck it for the moment.
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Oh, one more thing. I had the fans replaced by Apple two or three years ago under warranty. The right fan is rattling again, but RPM is consistent so it's not completely dead yet. I did find these replacement fans if you're handy w/ tools:
http://www.amazon.com/dp/B0055U8B1A...TF8&colid=2KMFHY9K1NWN9&coliid=I3OIF7AEV17LBR
http://www.amazon.com/dp/B003CO5KSI...UTF8&colid=2KMFHY9K1NWN9&coliid=IO172NU8YA4LI
If mine finally goes, I'll be replacing them myself.