I have an early 2008 15" MBP with 4 GB of RAM running Mountain Lion. It had been getting quite slow for Aperture and certain other programs, so I finally broke down and bought a refurbished 2012 Mac mini i7 quad-core. It was the best bang for the buck for me. I would have preferred a rMBP or a high-end MBA, but I couldn't justify the cost. I was going to sell my old MBP, but since it isn't worth all that much, I've kept it around as a back-up for when I want to be mobile. Eventually I'll probably get a new rMBP or MBA. So a 2007-2008 MBP is still usable, but don't expect blazing speed.
----------
Interesting. I was thinking of replacing the HHD in my early 2008 MBP with a cheap SSD. As I mentioned in my previous post, I've kept it as a back-up since buying a 2012 Mac mini. So I take it that you wouldn't recommend bothering upgrading my old MBP with an SSD?