When driving to Islay most people probably take the A82/A83 route from Glasgow. If you have a bit of spare time and the weather is nice I now recommend to take a detour via the A817 Haul Road (and A814) between Loch Lomond and Gare Loch / Loch Long. The views are fantastic, especially on the western part of the A817. The Arrochar Alps and other mountains line the horizon, a great view.
I had known about this route for many years, but for various reasons (in particular weather and time) had never got around to drive it. In June 2018 I had both the time as well as some decent weather, so I decided to go for it. Well worth it.