L Series is definately the way forward, my old efs 18-55 was so bad in comparison that i shot it after using an L, only downside was then i wanted a larger L series, then a camera with a full frame sensor and before i knew it i'd parted with 3 1/2 grand. The shots look the b****cks though but for general landscape/portrait shots with normal light i dont thnk IS makes much difference but handheld at 400mm would be virtually impossible without it, that said its rare to be in a position where a tripod is impractical but i like having the option to not use one, i guess it comes down to money really, an L Series IS lens will give better shots in most circumstances, what you have to weigh up is can you justify the high price.