OS X is a great tool for aerospace engineering! Preliminary assesments where I work show that high-end Sun and HP workstations are only marginally faster than a high-end Mac for our tasks, even though these workstations are at least three times the price of a high-end Mac. OS X's unix core makes it easy to port our analysis code over. Finally, I can optamise spacecraft trajectories on a laptop... perhaps from the beach. And I can run Powerpoint and Word on the same platform which is a big advantage over a Linux laptop.
I've used Macs, Unix, and Windows systems for years and OS X is easier for my type of work than any of them. I have a very nice GUI with OS X, but I'm not forced to use it when the command line is more convienant (say, for large batch jobs or for numerical work).
I love little features of OS X like being able to drop an icon into the terminal window and having the OS expand the full path to the file... sweet!
I ran OS 8 before upgrading to 10.0 on my G3, and I haven't had any major problems with OS X. I run Classis apps regularly with only the occasional glitch. I think people who are having trouble switching from 8 or 9 to OS X are having problems because they are trying to force OS X to be like 9, rather than simply spending a week to get to know the new OS.
You can never have a good relationship if you always compare the new girls you date to an ex-girlfriend.