I've been sitting on this one for way too long, partly because I wasn't sure how best to present it. Acids get a bad rap in public life, frequenty associated with mouth-stinging, flesh-dissolving and consciousness-tripping. 
 
Even in perfumery, I've had people make a confused or dismayed face when I suggest that they use a bit of Labdanic acid or 9-Decenoic acid. Yet these Carboxys- are, in almost all cases and concentrations relevant to perfumery, as harmless as it gets. 
 
2-Methyl Valeric acid is quite specific in its smell to me. At 1%, it smells like opening a bag of dried apricots and taking a whiff. It has that delicious dried fruit sulfurousness, paired with the sourness of fruit skins. In addition to this note, though, there's something dark, almost leathery like prunes and dates.