"More of a problem with 600 series is gummy shutter blades. When you pick up an old piece that was resting for a while, you may not see the problem immediately but you will soon see it after a few rolls of shooting. This is due to deteriorating rubber bumper in the shutter assembly"
Yes, my EOS 650 seems to have this problem. I (very) gently cleaned the gummy stuff off the shutter blades and, fingers crossed, it's been fine -- I ran a few hundred shots off with no film in the camera without it sticking and have done this every couple of days for a week or two, and the sticking hasn't recurred so far.