[Ron McGill]

Have recently upgraded to Canon 7D from 400D. Looking at upgrade of current Sigma 18-200mm zoom. Thinking about Canon 70-300mm L USM IS as compromise on cost and potential for sport and wildlife interests. Sigma not stabalised. Understand 70-300 does not accept extenders. Would be grateful for comments from 7D and 70-300 users.
Currently take images of landscapes (have Sigma f/3.5 wide angle), macro of plants, fungi using Canon f/1.8 prime with extension tubes, sport and rural action shots. Looking for telephoto for close up of plants (shallow depth of field), wildlife, but don't use hide or camp out to get full frame shots.
Is build quality of Canon L series preferable to Sigma 400/500mm super telephoto lenses?