Me either. This is the single best improvement I've made in the last 4 to 5 years. My MBP is awesome with this X25-M. I have adapted myself to use this little 160gb space very confortably using some external storage at home. This is great! I sure do recommend this to anyone!
I have an 80 gb intel x-25m in my 13 inch MBP but I also took out the superdrive and installed a 500gb live in an enclosure in the optical bay.
How does that affect heat and battery life? Does the second disk switch off when not in use?
Yep, it is off when it isn't and use. It does affect battery a little bit but I have found it be negligible.