Granted I am not as sophisticated with the exchange companies (visiting TUG is like dropping into a Physics convention for me), but the whole thing didn't seem to work well for my needs. I find that if I don't call at the 11 mos. mark for Disney rooms, I tend not to get the HA units I need. SSR runs out of HA studios fast, while 1bedrooms do tend to stick around a bit. Because of that, I've been very reluctant to even chance an II exchange into Disney.
Out doesn't seem an issue. I've never had a problem waiting for the month before check-in to call for a HA unit at Marriott. They have tons of them. I'm guessing demand is lower for HA rooms at Marriott than Disney. Perhaps it has to do with the local transportation issues. I know I can fly down to Disney and never have to worry about how I'll transport my powerchair around. But whenever we stay at Marriott, I know I have to provide my own transportation. That transport is significantly more expensive. The going rate for a HA van-accessible minivan is $100 per day.

And you can forget about relying on HA taxis except in the rarest locations like Vegas or London.