I guess I have two issues: one is the fact that there were big hiccups in the system during our trip. I really hope everyone who's saying that what we experienced was the exception is right.
The other issue is, as you say, how the MDX is supposed to work. I think whether you like or dislike the new system will come down to what kind of traveler you are. If you're the kind of person who plans the trip a year in advance and can't wait for the 180-day mark so you can make ADRs, then yeah, you're probably gonna like MDX. If you're the kind of person who wakes up and says "I know we talked about doing HS today, but I could really use a day at Blizzard Beach" then you're probably not going to like it as much. I fall into the latter group.
Maybe we're just lucky, but paper FPs always worked incredibly well for us. We never felt that we had to do a lot of criss-crossing the park; we'd get a FP and then do the less-busy stuff nearby, then go back during the FP window. In contrast, when we used FP+ on our last trip, we either couldn't get the rides we wanted, or the times offered were really inconvenient.
I would be interested to know the percentage of overall park goers who used the paper FPs versus the percentage who use FP+. FP+ is being heavily marketed in ads and on all promotional material, in a way that the old FP wasn't. It stands to reason that if there are more people using FP+ than there were people who used paper FPs, it's harder to get "good" FP+ reservations on short notice.