I also have a late 2008 MBP Unibody. I just received it back from my son, who needed to get a Win laptop for his scholl's engineering program. (For the stupid "ridiculous" commenter: paying for college tuition -- a good, not-ridiculous reason to continue using a perfectly usuable Unibody MBP! :^P)
I installed 8 GB of ram (after verifying the firmware was the latest version, supporting 8GB) and replaced the original 250GB HD with a 500 GB hybrid HD (small amount of SSD on front of the HD).
I then installed ML and Bootcamp Win 7.
Long story short: Absolutely fine -- ML is quick (not "quick enough" - quick). A 1st-gen Unibody MBP is perfectly ok w/ML.