I just installed my new Seagate Momentus 100GB 7200RMP harddrive in my Toshiba Tecra M4. There is a noticeable difference in speed when using the computer, especially in disk-intensive applications like video-editing apps etc. The computer boots faster, and applications generally loads quicker. The whole thing just feels quick and snappy.
The upgrade process took me approx. 45 minutes, using Acronis True Image to clone the disk. I chose the Seagate disk after reading this report comparing a few small SATA harddrives. Thanks to these threads on the Tablebuzz.com forums for hints and tips!