(This has become longer than planned... but hopefully it helps someone with the same questions I had before my purchase!)
Much praise has fallen on the Glenmorangie Sonnalta PX since its 2009 release, and it even used to be in the top 10 (or 3!) whiskies on Connosr. Uunfortunately bottles are increasingly scarce, so the decision to purchase one is harder. I wanted more input than 'thumbs up' or a list of flavors and so asked the Connosr forums: What was so special about this malt, at only 12 years old? What whisky does it resemble? Finally with a bottle, I happily sought to answer my own questions.
The fresh bottle gave an overall impression was of purple grape overtaken by spirity tannins. It was not unpleasant, but I felt let down because it was more serious than the rumored PX sweetness. After 2 months in an 80% full bottle, some of the tannins oxidized nicely to give more floral, fruity, and milky sensations. Even more oxidation seems to bring zesty pepperiness (Personally I think it is best before this happens). My advice if you get your hands on a Sonnalta (and others have agreed) is not to drink much immediately, but give it the time to open and reveal some of the fruitier tones (also after pouring). Here is my description of the matured bottle:
Nose: The overall impression is of elegant raisin cream with a refined tannin. At first, a grape theme is apparent: Concord grape pie, grape flesh, or raisins. This is combined equally with vanilla-cream and silky caramel. Of secondary prominence is that noticeably "deep and sharp" tannic overtone (like grape skins, or allspice, lemon rind, 100% cocoa powder, almond skins, or maybe amaretto). And thirdly, if you keep pulling, there is a floral smell: like rose petals or an overripe yellow pear.
Palate: Orange-zest entrance mellows quickly to caramel/rose/skim-milk. With growing heat, a tobacco-like amaretto sensation overtakes. Above this, a dry cocoa powder tannin comes in as a dominant impression. Vanilla-toffee tails into the finish.
Finish: That vanilla-toffee becomes a rose-like grape-iness, and the dry cocoa sharpness comes back, with potpourri. It is fairly long, with potpourri and that dry dark-chocolate/almond-skin/grape-skin tannin injecting some intrigue.
Overall: Back and forth between mellow (raisin-milk-vanilla-rose) and spice (cocoa-potpourri-zest). Like biting into ripe purple grape and alternatingly tasting the sharp skins and the sweet, thick inner flesh.
So is the Sonnalta special? Based on my limited experience with 20ish sherried unpeated malts, I would say "kind of." My mind has categorized 2 classes of sherried malts-- broadly speaking. The first are the fruity or "cherry" themed ones, which fall within a spectrum from apple-orange-cherry-peach-apricot-plum. For examples: various Aberlours, the Glenfarclas' 15/21/40, Macallan's 10 and 1824 series, Glenlivets 18/25 Glengoyne 21, Glenmorangie 25, etc. The second group are the "tannic" ones. I usually describe the feature as almond skins/100% cocoa powder/grape skins, or as dry, sharp, bitter, tannic, or sophisticated. (Others have termed it sherried smokiness, though 'steaminess' would better clarify the lack of peat.) For examples, Glenfarclas' 25/30, Macallan 18, Auchentoshan 3Wood, Glenmorangie Lasanta. These are much less fruity than the first group, and their nuttiness is more of a walnut tone.
The Sonnalta is a member of this 2nd group (perhaps most like Macallan 18), except that the tannic cocoa edge is milder and fruit is not absent (grape, the dominating one, is not characteristic of group 1). I have not previously identified raisin as the main fruit influence in a sherried malt; I even wondered whether liquid sherry was mixed into the Sonnalta. My feeling is that the Sonnalta is distinctive in presenting a character of raisins and milk to balance out the tannic forces. The Glenfarclas 15 also hit a middle ground, but I find it a bit more playful etc.
So is all the above enough to justify a purchase? If you can allay curiosity by imagining the sensations (based on my and others' reviews)-- or better yet actually acquiring a sample-- then it should come down to substitutes. For instance, I would not imagine that the experience is justified costing much more than the Signet or of the Macallan 18 Sherry. There is not the multitude of spices as found in e.g. Glengoyne's 21, and the palate and nose are not as dynamic as e.g. Glenfarclas 15. And looking ahead, I would guess it it should be easy to repeat the 12 yo Sonnalta creation in the future? But this should not take away from the fact that the Sonnalta is an Scotch, really smooth, tasty, refined, and pleasurable.