The thing with shells, or caps, is you need to weigh what you want and need, and what you dont want. If your REALLY want the MPG, then a cab will help, but I dont know how long it will take until it pays itself off. If your a surfer and you go all the time and dont know when your going to go, a shell is nice so you can keep your board and things in it and able to lock it. I think on some trucks it looks a little funky, and honestly, I dont like them. They have their purposes, but for me, I dont kneed it. I think it defeats the purpose of a truck. The idea of a truck is being able to throw shit in the bed carelessly and not worry about the truck itself. A bed is supposed to be all scratched up. If its a car your worried about your apulstry. I like being able to after school walk up to my truck throw my back pack in the bed and hop in and take off. My buddy has a shell on his tacoma. Either he has to unlock both locks, turn the handle and open the damn thing to put his back pack in it, or he has to open one of the doors and put his shit in and then get in the drivers seat. I think its a little annoying, but for camping, you can have some place warm to sleep. You can make a long trip through baja and not worry as much about your things getting stolen. But yet you cant put messy things like dirt or sand bags without getting the whole thing dirty. The shell makes it harder to wash out.
Just weigh everything and see what you want.