Damn! Space Shuttle is such a cool pinball machine, love it to death!
From a couple of years watching 'em come up and owning a Shuttle they have a few problem areas...
1) Playfield wear is very common, these games were rode hard and most didn't have mylar'ed playfields.
2) Plastic RAMPS - The playfield plastics are available new, but the RAMPS are not reproduced as I know of, and they are almost always warped even on nice machines I see. They are very thin and crack and warp like crazy up towards the top of the playfield. If you don't open the playfield carefully it also smashes up the ramp plastics at the top further risking damage.
3) Backglass flaking - These are NOTORIOUS for the art to be bubbling off the glass, especially towards the bottom where the game over, tilt, etc. lights are mounted along the bottom of Earth. There is a file floating around the internet to have a translight printed, but originally these were silkscreened mirrored glass. I've never seen a glass reproduction backglass, only the translights that pop up from time to time.
Overall electronically they are pretty reliable and I see more alive ones than dead ones. Any problems I've personally had with mine have been extremely minor other (soldering a wire onto a solenoid, flipper coil sleeves) . My game has a decent playfield but a wasted backglass. That is, until I found a guy who also collects coin-ops that worked an unbelievable deal on a near mint glass. The gas cost me more to go pick it up than the glass itself did!
Most of the mechanical parts/electronic parts are interchanable with other Williams pinballs of a similar era, almost anything that could break is readily available. The Space Shuttle plastic toy on the playfield is reproduced like crazy, all it has is a little frame and a switch mounted to it that you change over to a new one and then install the stickers.
I haven't been watching for the last year or two as much as I was in the past, but overall Space Shuttle gets lots of love and is a great addition into any gameroom with a super bitchin' theme instead of a corny one! The 1980's synthesized voice of the countdown and airlock warning are great! I especially love the "Aaaaahhhhhhhh!!!!!" electronic scream that happens every now and again when you lose a ball! The only "annoying" things about the machine I would say is the ringer/bell when you spell SHUTTLE or get a replay and if someone leaves a ball in the trough with the game still playing it makes a sci-fi oscilating bubbly sound that grinds on your nerves until the game is over and it goes back into attract mode.