Does the helmet compartment fit a full-face ski helmet?

Yes, the helmet pocket is sized for standard ski and snowboard helmets, including most full-face models. Very large downhill helmets may fit tightly.

Can it work as a carry-on?

At 32 L it fits within most airline carry-on limits, and the padded laptop sleeve helps on travel days. Check your airline's size rules, since duffels can be wider than a standard roller.

No. The fabric resists splashes and light snow, but the zippers are not sealed, so keep electronics in a dry bag in heavy weather.

How does it compare with a boot bag?

It carries boots and a helmet in one bag, while most boot bags skip the laptop sleeve and helmet pocket. It is a better travel bag but less specialized for boot-only trips.

What is the best way to carry it?

Use the top handles for short carries and the shoulder strap for longer walks. The strap is not heavily padded, so pack heavy items low and close to your back.