I did a recce with a friend who was doing the route last year, and it was a great ride as smutchin describes. The only other thing is, if you catch the wind on the way down the Lochs, you'll get it back on the undulating roads the other side.
The Schiehallion isn't horrific... but it's one non-stop climb. I wish I had trained so I could have enjoyed it a bit.
I'm a commuter and not much more, and only had 25 teeth on my cassette and felt like I needed another 5 (or maybe that training I was imagining?). I took a strategic stop to take in the view, ahem, but in my defense I had guards, a pannier rack, Marathon+ on my flatbar tank.
Descent was great fun.