Let's say all things being equal (the DM is a great guy, it's a good group of people, you have enough free time, etc.) . . . would you be willing to play in an AD&D 1e campaign?
I put myself down as a "maybe" for this one.
I only really started playing D&D proper with 3.0, although I had a basic box set from when I was younger. I have since picked up about a dozen AD&D books (including the core rules) and do have a hankering to run a one-off for my 3.5 group sometime. (I'm actually looking to include it within the same campaign world and semi-surprise them with it... but that's a story for another time!)
I'd certainly be up for playing a one-off, and might consider something longer under the circumstances. But to be honest, there's enough oddities to old D&D that I dunno if I could really being myself to play a full campaign of it. For a change of pace I could ignore stuff like level limits, XP progressions, multi-vs-dual classing and needing ADDICT to decipher the combat rules: some of those don't come up in one-offs anyway. But for a full campaign, it would need to be bringing something very funky to the table we couldn't do in another system in. Adding in some non-core books are only going to exacerbate things: I see more than one person has brought up Unearthed Arcana, and I'm sure some veterans will give us stories of how that book affected their game.
A classic adventure (either as one-off or part of a campaign) might be able to tempt me - playing through Barrier Peaks, the Giants adventures or original Ravenloft in their classic form, for example, sounds cool. There are 3.5 conversions, yeah, but dying in the
real Tomb of Horrors is quite a different accolade. ;-)
Perhaps a factor would be who was asking me. A GM who is keen on AD&D has the advantage of knowing the system and, well, being a keen GM - these do tend towards more fun games than when everyone agrees to a second best choice for everyone and the GM is only phoning it in. However, again, the big question would have to be why we're using AD&D rather than anything else: and if I got a edition wars spiel, I'd be pretty likely to say "Thanks, but no thanks". There's enough RPG tension in my life as it is, what with the internet and one or two friends around whom you can't bring up certain systems - I would be very wary of subjecting myself to more.