I’m using a MacBook Pro laptop as my primary development platform for work. The project has sprung for the OS upgrade to the new-ish release of Mac OS X, version 10.5.1, or “Leopard”.
So today I identified a hard disk I could erase, plugged it into an external USB case, hooked that up to the laptop, and cleared enough free space to accommodate the upgrade process. I checked “Software Update” and got the 10.4 install up to date. After a reboot, I started the upgrade. It restarted again and began the process. I left the whole thing on its own for a couple of hours. When I came back to it, I had a login prompt. It stepped through a wizard-style process for registering the upgrade, then rebooted one more time. At that point, I was back to usual laptop function, but now with Leopard running the show. Xcode is OS version specific, so that had to be installed from the DVD, too. There were optional updates for the standard Mac applications, so I installed those, too.
All in all, the indications are that it was a pretty smooth in-place upgrade.
I’ll have to see have my experience holds up following that. I certainly wouldn’t do that with Windows; whenever I do a Windows install, I take things down to a new partitioning and clean install.