I've tried the GlenDronach 15 years old
. While I think it has some lovely old-style sherry notes (balsamic, coffee, raisins, plums), there is also something dirty to it (mushrooms, dirt bin). Some may call that off-notes, others will like it for being 'different' (hence the good review on Whiskyfun for example). I personally have difficulties with it. Let's just say it's not the cracking dram that it would have been without those dirty notes. But don't get me wrong: at this price, it would be difficult to find better sherry bottlings.
From what I hear, the 18 years old is worse and contains some plain sulphur.
I can't compare to the older bottlings.