I have one in my T41p and just put one in my R40.
I did some informal comparisons of how the R40 "felt" before and after the upgrade and there was a MAJOR difference in responsiveness with the 7200RPM drive. Windows load time was signifigantly reduced and opening and closing programs was MUCH faster.
The power hit is very small, and more than negated by the time saved waiting on the HDD to do it's thing. My R40 still gets more than 5 hours on battery. (CPU set to adaptive and all power savings on)
In short, it made a sluggish R40 feel like a quick T41p in day to day HDD intensive tasks. People don't realize how much a HDD affects performance and can make a computer feel sluggish, or snappy.
BTW, according to SANDRA 2004 SP1, this drive SCREAMS and holds it's own among the fastest 7200 RPM drives out there.