I have grown to the point where I seriously distaste sherry matured whisky.
That said, a couple of weeks ago I got to sample Glenfarclas 40 and it was incredible. It was still a sherried whisky, no doubt about it, but the presence of sulphur and chemicals was almost nonexistent and the taste was deep and thick.
In fact, I was lucky enough to be able to sample several whiskies anywhere from 28-50 years old, mostly bourbon speyside malts. What I found, at least in my particular opinion, is that the most incredible whiskies I've ever had were between 30-40 years. There was a very noticeable difference. Under 30 and a whisky starts to regain its alcoholic roughness more and more. Over 40 and it's just an unnecessary price increase. The 50 year I had was great, but I'm not sure it seemed all that different from a 36 year old speyside I had. It actually had sort of a detracting hint of weird spoiled olives or something to it. Just some weird funky background that screamed "I was left too long in a barrel!" M
Personally I find sherry macallan to be overpriced and putrid.