Bruce - Jimboomba? Big fan of them. Almost had one myself.
My understanding of steel vs ali is that ali is 1/3rd the weight but half the strength for a given cross-section. So to get the same strength as steel, you need to use twice as thick a piece, but you end up with only 2/3rds the weight. That's not taking into account any stress-related issues, fatigue, etc.
Personally, I'd have no porblems with a aluminium-bodied trailer on a gal chassis. Jimboomba is the one that comes to my mind - they've been making them like that for years. You'd think if they had serious issues they'd cop a lot of returns, and probably be forced to change their construction. I've certainly not heard of anything - all their customers that I've talked to seem to be very happy.
Like anything, it comes down to how hard you intend to beat on it. I've got a steel bullbar, because I want it to be able to take a few knocks and still be servicable. I'm happy with the weight and fuel-economy taxes that I need to pay to get that. My father-in-law is looking to get an alloy bar because he wants the lighter weight every day, and is happy to have it save him once and then get replaced. Horses for courses. Not saying you necessarilly need to baby an alloy trailer, but just be aware of the fact that the lightness/anti-rust benefits come with their own special sets of requirements.
All that said Rockman, no, I have no personal experience with aluminium, and I don't work with it much at all, so can't answer your original question in any way, shape or form!
Thanks!
Matto