Thinking of taking the plunge within six months to a year. What should I look for (or avoid) with something used? How about first generation stuff? Parts still available for service/repair?

It's not the sort of thing I'd buy used, unless it came with the used bike I was also buying. I'd love to have Di2. Never had it, not willing to spend the bucks for it right now, but when I do, it'll be a new one. I don't want my first experience with it to be bad because I bought someones worn out Di2.

Yeah. I'm considering buying a used bike with Di2 gear. There's a nice-looking Giant Propel with Ultregra Di2 not far away. Just don't have $1400 to spare at the moment.

1st generation? I wouldn't. 7970 Di2 is pre-etube, and mutually incompatible with everything after. Lots of the stuff lying around....since it isn't wanted as it doesn't work with anything that came later.

Mostly the same as mechanical stuff- look for any damage, make sure everything functions well, and you should be in good shape. If you want extra piece of mind, take it to shop that's competent with Di2 and have them connect it to a computer and make sure everything is reporting correctly and up to date on firmware. Clarifying what Marcus_Ti said, 7970 is the only one to really avoid, 6770 (first gen Ultegra, second gen Di2) uses etube, which means wires and other parts are readily available, and is bueno if you're ok with ten speed.

Yeah, they do look pretty cool. The old Propel had awful gear cable routing though, so I'd only consider one in Di2... Also IIRC there was a recall on the brakes, which were somewhat improved, but of course not every bike had them changed.