I have the BakFlip Fibermax and I'm so glad I got a BakFlip for my Ram. I love being able to take my family camping and being able to stop at a restaurant or something on the way and not worry about coming out to find all my stuff missing. And with the power locks on the tailgate I don't have to worry about using a key to lock and unlock the back, which makes it much less likely that I'll forget.
The quality of the BakFlip is great and I didn't have any issues installing it. My 9 year old stands on it all the time and it is no worse for the wear. A little bit of water gets into the bed when I run my truck through the carwash but not much (although I did cover the stake pocket holes), so my stuff also stays dry.
I also have the Mopar Bed Extender (pic below) and have been pretty happy with it. The main reason I had the dealer add it on was for groceries. I put some carabiners (for some reason I had them removed when I took the picture below) on it so we can clip our plastic grocery bags to them and drive home without all of our food scattered throughout the bed. The nice thing is that it's really easy to take off so if I ever need to use the entire bed for something it's not a hassle.
Since I don't have a Rambox I really can't speak much to that, except to say that I secretly have Rambox envy. When I bought Elsie the trucks in my area all seemed to be equipped in one of two ways: with Rambox and no air suspension or they had air suspension and no Rambox. I chose the latter though I sometimes wish I'd have gone with the Rambox and no air suspension. I think it would be really handy for jumper cables and all that misc. stuff that everyone should keep in their vehicle. For that I added the BakBox to my BakFlip and it does a great job but the Rambox would be more convenient since I wouldn't have to hear my son laughing at me as I try and climb into the bed of the truck.
My crew cab does have the little storage areas under the rear floor mats and they will certainly store handguns in a pinch, though the don't lock so be careful there. I imagine they could also be used as coolers but I don't think they have drain plugs so I guess you'd have to get the water from the melted ice out manually.
EDIT: I just looked at the floor storage compartments and realized there's actually a little bucket inside the storage compartment, making it easy to dump out if it's full of water. Learn something new everyday...