I've ridden the Icefields Parkway twice, both in August, and although there were lots of tourists in Bannf, Lake Louise & Jasper, I wouldn't describe it the way Machka did. Booking in advance wasn't needed, the campground at Lake Louise was pretty full but the rangers were doing a great job helping cyclists share sites, so it wasn't a problem w/o a reservation. Other campgrounds were not full. I stayed at a hostel (Rampart) and there were only 6 other people there, all of them cyclists.
The scenery is spectacular.