I have a 2007 424-LTD, which is all koa. I used to have a 2006 first-year GS-K (koa/spruce) which would come to be called a K16 using standard nomenclature. Taylor has wonderfully consistent build quality, but koa wood can vary quite a bit in its properties. The more highly figured, the more likely that it will not be stellar tonally. I have encountered some $5K koa Taylor guitars that were not worth $500 tonally, in my opinion -- they sound like plywood but but certainly looked spectacular. That is a convoluted way to say, "play before pay".
On my 424 they used the fancy stuff on the top, and more pedestrian looking plain koa for the back and sides. As a rule, koa as a top wood is a bit brighter and crisper in tone than spruce tops. It takes longer to open up too, being a harder woods than spruce. As a body wood, koa is similar to maple. If you like the modern Taylor tone profile better than some other brands, all-koa is that character on "11". Full Disclosure: I have not played one of the new 724's in person, just speaking from experience with other models and other brands in koa.