I went the opposite route - I started off with metal water bottles (military surplus, sigg, etc.), then went to hard nalgenes, and now use wide mouth platypus reservoirs. They're about as easy to scrub out as a nalgene, not as heavy as a metal bottle, and they squish flat when not in use. I've also used them as floats in my backpack when canoeing. They are compatible with my MSR water filter, too.

I don't heat anything in my canteens. I use a quart-sized titanium pot for that.

The only disadvantage I can see with the platys is that they develop a smell after a few years. They're arguably not as durable, but they've held up through a total of months of bushwhacking.