Pros: keeps the hat's proper shape and size, reduces chance of wrinkling
Cons: not the smallest tool in the laundry room
Some people buy ball caps as souvenirs, some buy them as collectibles. Some even buy them to wear: what a concept, eh?! I fall into both the first and third categories, I guess - I buy them as wearable souvenirs. If you have a solar panel on top of your head as large as the one I'm sporting, the wearability of the souvenir becomes very important in sunny climates and at high altitude; which explains why I have at least three hats advertising different locales in Colorado... Wearing a cap, especially one you've become fond of, does tend to have a downside: as your head sweats, the hat gets grodier and grodier, until it begins looking as if it belongs to someone pushing a shopping car filled with salvaged trash and mumbling incoherently to an unseen companion situated somewhere above his left shoulder. When that happens, there's usually not much to do but fling the cap in the trash and go dig another one out of the closet.
Unless, that is, you have a Cap Washer by Perfect Curve. In that case, you merely wash it - the Perfect Curve Cap Washer holds the cap in its proper shape through a wash and dry cycle, even the curved bill of ball caps and visors. The cap comes out clean, though sometimes a little faded, and otherwise none the worse for wear. I've been wearing the same three or four baseball caps for years, and every one of them has been through the wash at least a couple of times. Even the ones with cardboard in the brims hold their shape, as long as they're allowed to fully dry. Assembled, the cap washer looks big enough for Paul Bunyan, at about 9"x14" and 6" high.
What is the Perfect Curve Cap Washer? It's just two cap-shaped open plastic frames of two different sizes. Put your cap inside the larger frame, put the smaller frame in place inside the cap, snap the two halves together, and - this is the fun part - place the assembly, cap and all, in your dishwasher! The dishwasher detergent works like a charm! As long as you don't use a heavy-duty cycle and do air dry, the caps don't even shrink, and my experience has been that both leather and plastic adjustable bands stand up quite well. Perfect Curve also says their cap washer can be used in washing machines, but I've never tried that route.
For an investment of four or five dollars, you can keep one your favorite gimme cap or an expensive "Lid" around three or four times as long - at least until you lose it, anyway. The manufacture is pretty crude - just injection-molded plastic frames held together by three friction points (the main reason why I don't put mine in the washer), but it serves its purpose just fine. If you've been tossing baseball caps that have dark sweat stains around the band, now's your chance to economize.