The 5 yo is the big seller, and it is promoted as a long drink and for its "mixability" - I recall an ad campaign in the late 1990s recommending folks drink it topped up with apple juice. Primarily targeted at European markets, especially Italy. A bit too "young" and lacking in complexity to appeal to traditional maltheads for drinking on its own or with a splash, I fear. But correct me if you think I'm wrong!
Yes, Glen Grant 5 is big in Italy. You will find it in ANY bar, even in the most remote places.
But it was never advertised as ideal for mixers or as a long drink (at least in Italy). I still remember the TV commercials some years ago with some guys savoring it straight: very unusual, since on TV commercials we were used to seeing blends with lots of ice.
As a matter of fact, the only other single malt of which I remember a TV commercial in Italy is the Macallan 7 (in which they spoke of 7 years as an unusually long maturing time... ROTFL!).
Anyway, Glen Grant 5 is decent: dry, a bit spirity, slighlty fruity, very clean. Not complex or memorable, but very drinkable and somewhat refreshing.