Yeah they seem like a cool idea. I saw them at 4 wheel parts a couple weeks ago. Wish I had a good enough rear bumper to hold the weight, but after breaking the hinge with just a 33, going to a 35 and 5 gallons seems like it's pushing it.
I wasn't that keen on them at first but after using them for a while I like them. Mainly the size and shape and ability to lock them together. I haven't put them on my spare tire, I have them behind the seat in my Ford.