The most surprising response I've had is none at all. I had heard the horror stories of boat captains, shop owners, instructors, dive site operators, etc. refusing to let divers dive with vintage gear. But for the most part, I've had no problems whatsoever. Last winter I dove my AK at Dutch Spring, on several boats in Pompano Beach, and even Ginnie Springs (in open water). I've been diving my DAAM and my Healthways Gold all Spring and Summer at Dutch Springs and in the Thousand Islands with equally little fanfare. My latest setup includes double steel 45s with a vintage ScubPro manifold and the Healthways single stage DH reg. I carry an AL30 pony with inflator, 1 second stage, and a gauge to mitigate anyone hassling me about no LP accessories or gauge on the doubles, but nobody has even mentioned it as an issue. Likewise, I have been using a horse collar BC with my wetsuit and no bladder at all if I wear a drysuit (using the suit for buoyancy control). Nobody has had any problem with it.
The only comments I have received from anyone who has actually even noticed the DH regs go something like "Is that a rebreather?" to which my usual response is something like, "look: no canister, no counterlung, no O2... so no, not a rebreather. But look how cool this 60 year old regulator is... wanna try it?" No takers so far. Every once in a while I get a comment from someone who has a DH, but is not actually diving it. Maybe this will encourage them to break it out and have some fun with it.