For me, I was somewhere between 8 and 11 years old, and was leaving a military hospital after a basic check up (I'm a military brat). Twas a quiet day on the air base, and as me and my dad were walking back to the car, we saw over in an adjacent parking lot, this large, all-black semi-truck with the big SGI cube logo on the side of the trailer, in silver. I had to go have a look, so, begrudgingly, my dad walked me over there to see if they'ed let us look inside. Sure enough, they had no problem, so we walked up the stairs into one side of the trailer into a fully carpeted room with a whole slew of SGI systems on a table, all running demos and the like. Couldn't touch anything, but I remember they at least had Indy and Indigo2 systems up and running. Spent about 5-10mins just staring at systems I thought were totally out of this world (all I had at home was an IBM PC/XT that booted MS-DOS 3.0 and a Commodore 64), then left and headed home.
That's all it took, and I've made it my life goal not only to acquire as many SGI systems as I have room for, but to try and get a few of them to run and do things. Throw in getting to use an Indy at Epcot Innoventions in the mid-90's, the Nintendo 64 being a joint Nintendo/SGI thing, and my childhood can be partially defined as having an SGI obsession.
Running IRIX on them is easy, but that's a dead OS now (I know a lot of people here don't like hearing that). Unless SGI/Rackable dumps the source code at some point (support ended Dec 2013, so...), it's going to get progressively harder to try new things on these systems. So...Linux! Which is not easy, because I am not a kernel hacker and it can take me days of puzzling over hardware documentation to get something to work that more seasoned programmers could do blind and with one hand.
I only recently started to resurrect Linux on the Octane, and that was tough to try and decipher how the HEART ASIC works, how to update Stan's old code, and how to wire in new Linux subsystem changes to make it all tick. Still stuck on getting SMP to boot, but all in time. Linux/MIPS has been somewhat taken over by commercial interests lately, and there just doesn't seem to be a lot of the old hobbyist crowd around anymore. So maybe I can put some spark back into that side of things soon. I still have to unify portions of the IOC3 driver with the IP27 side, and try to send in patches and not get shredded on the LKML over it. Then, big maaaaaaaaaybe, try to tackle IP35 support.
When people puzzle over why this interests me, I ask them if they've ever met someone that collects old cars. That's when the light bulb comes on and they understand my love for these old machines.
PS, in terms of going for SGI rarities, I acquired this off of eBay recently and hung it on the wall above the Tezro: