Is the Spartan worth it over a cheaper Swiss Army knife?

The Spartan sits in the middle of the range and gives you two blades plus openers, screwdrivers, corkscrew and awl in a light body. Cheaper two-layer models drop several of those tools; the Spartan is the better all-rounder if you want one knife for many jobs.

How does it compare with the Huntsman?

The Huntsman adds scissors and a wood saw in a thicker, heavier body. The Spartan is slimmer and lighter, so it carries better in a pocket, but you give up those two tools.

No. The Spartan uses a slip-joint backspring, so the blade can fold under pressure. Use it for normal cutting, not prying or heavy force.

Can it be opened with one hand?

No. The blades and tools are opened with a nail nick, which takes two hands. Victorinox makes one-hand models if that matters to you.

The knife itself, with the toothpick and tweezers fitted in the scales. It does not include a pouch or sheath.

Is it legal to carry in the US?

It is a non-locking folding knife with a blade under three inches, which is legal to carry in most US states and cities. Check your local rules, since some places restrict knives by blade length or type.