I went in June a couple of years ago, around Fort William and didn't think the midge problem was bad, until we had a wet day followed by a sunny day and I decided to fish on the loch. I literally got eaten alive, after an hour I had to stop fishing and my face and the back of my hands were coverd in red blotches.
Used mossie spray after that even though its evil stuff.
Don't let it put you off though it's a brilliant place, I will definitely go back.