This info is a few years old, but we had good luck with the reservation BMA system. I was tipped off by a buddy to a good ranch that took reservations starting on the 29th of August. I was able to get through on the afternoon of the 29th and got one of the last spots available. We had a great hunt and my buddy and I both killed good bucks. The folks managing the ranch were great (they were hunters and were as excited to see what we killed as we were- so we got some help retrieving both deer).
That being said... I think we may have gotten really lucky. I tried getting through on the 29th and no one was answering the phone. The ranch was huge, and split into several parts (4 I think) and the hunt reservations were only for a 5 day period. Considering the length of Montana's season x 4 areas... They would have to have been taking calls all day to fill up their calendar legitimately by that afternoon. That means that most of those time slots were most likely "pre-reserved". Also, the best areas (from what we could tell) were already taken as well. My buddy killed his deer on our 3rd day in the middle of a -20 blizzard. The next day the ranch manager helped us get his deer out (mostly because he wanted to see it) and then told us that all of the other groups had gone home because of the weather. Since the ranch was empty, he sent us to a basin where he said the deer would congregate in this kind of weather. That's where I killed my buck.
Like I said, it all fell together for us, but I think we got really lucky.
And... I just checked... That BMA isn't registered anymore. I think the ranch might be a trespass fee place now....