I started with HP-UX 9.03 on a PA-RISC-powered 715-75 (to use Emacs, our whole research group logged into the 735 server to edit there, which was faster than running it locally).

Any unclean pointer fiddling in C, and the process was terminated by the OS, so the machine was wonderful to use as a development box (especially with Purify installed) for software that would later be run on Windows or Linux.

I eventually bought my own refurbished (and using academic discount) 715 (instead of a car), so I had the fastest machine in our student dorm of anyone I knew, undergrad, grad student or professor. I could just write my Master's thesis when everyone else kept re-installing Windows - the HP never crashed in 6.5 years, which has left me with deep respect for the old-schol (pre-Compaq) HP engineers. The machine (21" color CRT) occupied half of my 9 square metre dorm room, but it also kept me warm.