I did the loop in August a couple years ago. Lots of day hikers on the northern side of the loop close to the trailhead (I assume the southern side was too steep to attract day hikers). Then a decent number hikers again once you hit the JMT. Even so, it was never so crowded as to be unpleasant or to make it hard to find nice campsites.
I also second (third, whatever) the recommendation to detour into Sixty Lakes basin. If I go again, I'll also detour in the other direction from Rae Lakes, and check out Dragon Lake.
Also, as an added perk: when I went, the Roads End ranger station rented Bearikades for something very cheap. I'd planned on taking two bear cans, but my girlfriend and I were able to rent a single expedition which, together with a separate food bag for the first day and some skimping, worked for the whole trip.