I already did such a thing back when the MBP was new. At that time, I created a partition for the system files and another for the user data. However, that setup was not too optimal and, when I got the 7200RPM hard disk drive six months later, I reinstalled again in a single partition. Just for convenience.
But external fragmentation hurts performance a lot, specially in my case because I need to keep lots of small files (the NetBSD source tree, for example) and files that get fragmented very easily (sparse virtual machine disks). These end up spreading the files everywhere on the physical disk, and as a result the system slows down considerably. I even bought iDefrag and it does a good job at optimizing the disk layout... but the results were not as impressive as I expected.
This time I reinstalled using the following layout:
- System: Mounted on /, HFS+ case insensitive, 30GB.
- Users: Mounted on /Users, HFS+ case insensitive, 50GB.
- Windows: Not mounted, NTFS, 40GB.
- Projects: Mounted on /Users/jmmv/Projects, HFS+ case sensitive, 30GB.
Using this layout, the machine really feels a lot faster. Applications start quickly, programs that deal with personal data such as iPhoto and iTunes load the library faster, and I don't have to deal with stupid disk images to keep things sequential on disk. However, the price to pay for such layout is convenience, because now the free disk space is spread in multiple partitions.