I inherited a lot of vintage audio gear from my dad, including over a dozen RTR tape decks...and fortunately, some tape to go with them. I've gone through most of the decks to some degree - a few pretty nice ones in the bunch, although no Revox or Otari. Some even have tubes, including an Akai M8.
Most of the refurbishing I've done is cleaning the linkages, replacing belts, and even a couple of kits with new caps, etc. My current project is an Akai X-1810D, which also has an 8-track player incorporated. Apparently a mouse enjoyed this one, so there was a fair amount of cleaning needed inside, along with mending a few chewed wires.
Anyway, one aspect that bothers me, and I'm surprised no one has mentioned it here, is the motor/fan noise. I think every single unit I've touched has some degree of noise from the capstan motor, most of which also have a small fan attached, that adds a whooshing sound. It's a real deterrent for me, even though I enjoy the process of using the decks, including making recordings of records. Some of the service manuals show lubrication points for the motors, but they are often not easy to get to, and even if I can access those points, it doesn't seem to help all that much.
I guess some of the nicer models mentioned in this thread must be very quiet, because I can't imagine any 'audiophile' tolerating such noise.