It's been great. I've installed in on two Core Duo machines from 2006 (MBP and Mini) and they feel like new. I seriously noticed speed increaes on these two machines.
My Santa Rosa MBP was already snappy, and didn't notice much difference. I installed it also on my wife's Mac Pro (2006 2.6 Quad) with 6 GB of RAM. Didn't see much difference there either, but I don't use it for the heavy lifting that she does with it (Photoshop, Aperture, Illustrator, InDesign, etc.)
No stability issues, and works great on my machines. I had some issues on a few upgrading, but that is a different story. 🙄