I wanted an SSD in my aging MacBook Pro, but I didn't want to spend several hundred $ to get a big SSD drive. The solution? Buy an optical drive bay SATA enclosure, a small(ish) Intel 120GB SSD drive, keep my big 500GB Seagate hard drive, and get the best of both worlds.
I've opened the 2008 MBP 15" before, so the whole process didn't take more than 45 minutes. I'd recommend anyone to keep good track of all the screws. Use the iFixit screw sheet and you'll be fine.
I decided to put the SSD in the drive bay and the 500GB hard drive in the optical bay, reasoning that the SATA port in the hard drive bay would be faster than ATA port in the drive bay and the ATA/SATA adapter in the iFixit enclosure. And the main point of this upgrade was to get as much performance as possible from the SSD drive. You should check the height of the hard drive before you attempt this, but mine seemed to fit almost perfectly.
After connecting the keyboard and confirming that both drives showed up, i reassembled the laptop. I proceeded to use SuperDuper to transfer the content of the hard drive to the new SSD, excluding the biggest folders - music, movies and pictures.
After completing this, I set the boot device to the SSD drive, and restarted. The speed increase was dramatic, and well worth the upgrade. The laptop booted in 15 seconds, and was logged in and ready to go in 3 seconds. Apps launch instantly, even web pages seem to load faster (maybe quicker access to disk cache?). Everything is screaming fast. Unless something happens to it, I plan on keeping the laptop for some more years.
I decided to keep the music and movies on the big drive, and just made symbolic links to the folders and this seems to work without problems. iTunes seems much faster even though the music is still on the "slow" drive.
Battery performance seems quite similar, although I haven't put it through demanding tests yet.
If you've been holding out on buying an SSD because of the price, I wouldn't think twice about this upgrade. I haven't had any problems so far.