My Thousand Oaks solar filter arrived the other day and I got to try it out on a rare morning when we weren't clouded over. Seemed to work just fine and passed its first test for the Aug. 21 eclipse. Handheld, ISO 400, f/5.6 at 1/1600. Manual mode except for autofocus, which the 5D3 did flawlessly on live view. There is even a very small sunspot visible at 10 o'clock about 30% in from the rim, confirmed at the Spaceweather site the same day.
I'm not going for high class solar scope type images, I just want to experience, and also shoot, a total solar eclipse this summer.