Currently I have the Canon 75-300mm USM and am thinking about upgrading to the one with IS. Does the IS lens produce far better images than the other one? Is the IS lens also soft at full focal length? I would like your opinions and experiences. I know some of you would suggest I should probably go with 70-200L lens but I don't want to spend more money right now. I'd rather buy a 400mm L lens down the road. Thanks for your feedback.