I've got a Rubbermaid deck box by the pool, a small shed for the kids bikes, and a horizontal shed that I keep the garbage cans in. They hold up great and are easy to keep clean. Pretty easy for someone to break in to if they want, but I don't know how much more secure the dock boxes are.
I picked up a great deal on a Black Friday Sale at Lowe's on a Suncast Horizontal to keep the new portable generator in. It's still new, but looks like it will hold up as well as the rubbermaid stuff does.