My mid-2009 MacBook Pro was starting to feel a bit old around the edges but we're on a multi-year replacement cycle at Yale and I wasn't going to get a new one soon. So I instead opted for an SSD drive upgrade/replacement.
I got a new 750 gigabyte (rotating) hard drive and used the MCE Tech OptiBay to mount an Intel 160 gigabyte SSD. So I have around 900 gigabytes of online data and my system and application boots are around 4x faster.
I used "Trim Enabler" to enable TRIM support for my Intel SSD on Mac OSX 10.6.8 (updated 2011.08: and MacOSX 10.7 Lion).
INTEL SSDSA2CW160G3: Capacity: 160.04 GB (160,041,885,696 bytes) Model: INTEL SSDSA2CW160G3 Revision: 4PC10302 Serial Number: CVPR11xxxxHF160xxx Native Command Queuing: Yes Queue Depth: 32 Removable Media: No Detachable Drive: No BSD Name: disk0 Medium Type: Solid State TRIM Support: Yes Partition Map Type: GPT (GUID Partition Table) S.M.A.R.T. status: Verified
Highly recommended for all MBP owners -- if you're willing to give up your optical drive bay!