I've never seen one break, and we had plastic buckles on all the rental gear that was in the water everyday over the summer, that being said I don't want to have mine be the first one I see... on the first day of a long diving trip 

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Scubaroo:Has anyone ever had a plastic buckle break when it wasn't like, frozen brittle from cold? I used to have one brand of SS buckles that scratched the **** out of tanks, I used to line the tank with ducttape to prevent it. I'm currently using plastic buckles with no problems, but I have easy access (funny about that) to a new brand of SS buckle that look like they won't scratch the tanks. Thinking of switching, simply because it could mean another 1-1.5lb of integrated weight on the rig (they're heavy buckles - each strap weighs about 13oz.).
OTOH I wouldn't want SS buckles on a travel rig - there the weight is working against you.