I know that you're supposed to turn IS off when you're using a tripod, but I seem to be getting blurrier pictures at slow shutter speeds with IS ON! I tried it at various focal lengths with a shutter speed at around 1/30 with IS on and off, but the pictures with IS off always turn out better. When the IS is on, I focus first, lock it for a couple seconds to let the IS "kick in" and then depress the shutter all the way.
When I make the shutter speed faster, the problem seems to go away and I can get sharp pictures again with IS on or off.
I attached a couple pictures, let me know what you think.