Some lenses have IS, some don't. When is it necessary to fork out the extra money and go with IS? Is this mostly important with zooms? There is about a $1K difference between the 70-200 2.8L with IS and the one without. Is IS crucial on a lens like that? What about the 100mm 2.8 macros? The L IS lens is about $450 more than the non L IS lens. Is there a HUGE difference in performance and is it strictly related to the IS? Also, does it matter that there is no IS on the Canon EF 24-70mm f/2.8L USM? The 24-105mm 4 L has IS but is it any better than the 24-70? Does the lack of IS matter on a lens like the 85mm 1.8? Sorry if that's a lot of questions! Just trying to get my ducks in a row! Thanks for any thoughts/opinions/advice you may have!