It's my understanding that Don A) is not good about replying to emails, but will pick up the phone B) is ridiculously busy, and probably frankly swamped C) seems to be better at dealing with and helping people who call and talk with him a lot.
He is so highly regarded for his work (and rightly so) that it seems he can pick and choose whom he helps and how fast. I imagine he works at a pace that is sane and functional for him first, and his clamoring customers second. I suspect he works faster for folks with whom he has a good relationship- be-it personal friendship or heavy return customers- or else for people whom he wants to get out of his hair. I fall into neither category. Last year I sent him a lens which had been dropped. The helical was frozen, and it might have become a paperweight. To my surprise DAG fixed it; it took three months from start to finish. Most of my emails went unanswered- but he always spoke with me when I called on the phone. I was hesitant to take up his time with phone calls, but my impression was that if I'd called him regularly, one way or another it might have returned faster.
Sherry does fine work, and I'd happily send her my gear. I've had lenses and bodies that she has worked on, and never a problem with her. But Youxin Ye is the first person I'd deal with for work on older bodies. He does excellent work, is very fast, provides excellent communication, and is VERY reasonable in terms of cost; he's down-right cheap, in fact. Leica work is a side project for him, not his main job, so his overhead is lower, I presume. However, it's my understanding that he only works on older classic gear, and won't touch cameras with electronics.
