I've reviewed this whisky once before, but that was from a dram at a bar, so I didn't feel completely confident about my assessment. Subsequently I picked up a 3x200 mL pack that included Glenkinchie 12, Oban 14, and Dalwhinnie 15 - the Classic Malts Gentle Collection. While I've found that too often 'gentle' is another way of saying 'bland' in the whisky world, I wanted to give it a fair chance.
Once again, I'll leave the history to Malt Madness, as it's fairly complicated. A few noteworthy things about Dalwhinnie is that the distillery is located at a higher altitude than almost any other in Scotland and that it still uses very old-style worm tub condensers, which contribute to the flavor of the spirit by reducing the amount of copper contact. As with most of Diageo's single malts, this one is reduced to 43% and chill filtered.
Dalwhinnie 15 Year
Nose: sweetly malty and grassy, lots of vanilla and fruity (raspberry) esters, very light sherry, barest hint of peat/vegetation, cotton candy, grape (purple?), apple cider, dates, graham crackers, cinnamon chocolate, and honey. After adding a few drops of water, it becomes flatter and more vegetal and oaky, the malt and vanilla notes become mustier, the floral notes increase significantly, and it become distinctly purple rather than grape-y.
Taste: sweet opening - malt, vanilla, and very mild sherry, moderate pepper and ginger mid-palate, light oak tannins at the back. After dilution, it becomes even sweeter with more malt and vanilla, there's a pleasant sour apple and wine tinge throughout with a hint of cacao and something grassy, the pepper is reduced, but the tannins become sharper and more pronounced.
Finish: oak, pepper, sweet malt and vanilla, a tiny bit of sherry
As Serge noted, this is a great malt for bringing people over from the blended whisky side. It's not very aggressive - I don't think gentle is the adjective I would use, but pleasant seems right. There isn't a lot of complexity, but everything present is solid and tasty. I also think it accomplishes a lot of what Diageo is lacking in the Lowlander department - a lighter, grassier spirit that still has enough heft and flavor to be enjoyable. It also improved over time, which is always nice. While I'm not sure it's one that I'm going to buy a whole bottle of at this point in my whisky journey, I would still be quite happy to drink it if someone gave me a bottle as a present.
Best of all, it's very reasonably priced, clocking in around $55 most of the time. Given that a lot of 12 year old single malts are now starting at that point, it's quite a bargain for a 15 year old. There are many things to dislike about Diageo, but they have been keeping the prices of a lot of their entry-level malts under control, which is appreciated.