The 15mm is stock on the LS models with dual Brembos up front.
It provides very strong stopping power, especially with stainless steel braided lines, but the relative small distance of lever travel from actuation to full-on lock is pretty small and takes some getting used to.
I may not change mine out, but I have thought about using the 13mm (I think this is available) bore M/C would give more travel and a more progressive "feel". I know that one is not supposed to ride with just 2 fingers on the front brake, but if I use more than (2) on my LS there is a greater tendency to "over-brake".