While I agree that it is a great place to climb all Summer long, Middle Sister offers two very different mountains depending on the time of the season. I've climbed in the Sisters area in May and in August and hope everyone is perfectly clear that scree happens as the season wears on. I know it makes the mountain more accessible but I recommend an earlier, snowier, less-dusty climb in late Spring or early Summer. Plus you get the real Alpine feeling if you still have the snow under your feet.
If you go in May, be ready for wind and freezing cold (we had 19 degrees at camp in the trees) and a total snow, alpine experience. On the other hand, I went in August last year and never even set foot on snow the whole way! Two totally different mountains. But if you consider yourself an alpine climber go early. If you are a hiker who wants to bag a summit, go later in the season.