I have two 18.5 WSM and bought a Battle Box through Kevin (smokinit). In addition to the added cooking space, everything is just easier. Easier to cook multiple briskets, ribs etc. No more having to remove one rack to get at the bottom rack, no more having to add more water during a cook hoping it doesn't splash over on the coals, no more having to prop up a full packer with a soda or beer can until it shrinks. I wish I had made this move sooner.
I still have the WSMs. My wife, who had never used them before, now uses them to make wings and appetizers.