I tasted this at the end of a brilliant evening of whiskies and sherries, so my notes may be slightly more euphoric than is strictly justified.
Whiskybroker Springbank 15yo Rum barrel #300 52.8%
The nose is light and smoky. Perhaps brown sugar? It's sweet, but sweet without being syrupy - it is noticeably light in body. Fresh and maybe a wee bit salty.
With some water it becomes distinctly metallic; iron (which I often find in whiskies from Campbeltown) and aluminium. I should explain that when I find iron in a whisky that's a good thing, whereas aluminium is nearly always a bad thing.
The palate is very light and sweet. There's that lovely Campbeltown smoke in the finish. Just delicious.
Adding water doesn't change the palate so much. It remains soft, rounded, and smoky. Perhaps a little peppery note creeps in.
Conclusion: This is just ace. I'm a Springer fan, but even if you're not I think you'd find this to be a superb whisky. I'm not sure how much sweetness the rum barrel added, but it was certainly a good cask.