Yep, and in the Gifford's shooting, tackling was accompanied by somebody slamming a chair into Loughner first, IIRC, undoubtedly slowing his reload.
You probably can't decide to attack after you realize the shooter is reloading and have high expectations of being successful. That decision likely needs to be made in anticipation of the reload so that it may be launched as soon as the reload is recognized.
"If you look through your scope and see your shoe, aim higher."
-- said to me by my 11 year old daughter before going out for hogs 8/13/2011