A good pair of trauma shears is a must-have item for your kit, since you'll sometimes need to cut away clothing to get to wounds. For the layperson, there are three main options: a cheapo set from Amazon (Madison Supply in my case), the XShear shears, and the Leatherman Raptor shears.
Which is best? Well, I grabbed some old t-shirts and pitted all three against each other in a no-holds-barred cutting test to find out...
Amazon's Choice (Madison Supply) - $12
Honestly, even the cheap set did fine. While the hard squared-off plastic handles aren't terribly comfortable, and the blades are made of thin flimsy steel, none of that matters if you intend to use the shears for a one-time emergency. The Madison Supply set I bought didn't have any problem going through clothing, which is probably all that a non-first-responder needs. This is what I would take on an airplane, since I wouldn't be out much if an overzealous TSA agent confiscated them.
XShear - $40
XShear comes from Wes Brubaker, a paramedic and flight nurse who wanted something better than disposable shears but cheaper than the Raptors. The XShear slots in right between them pricewise, but it still has heavy duty blades, comfy rubbery handles, and a massive center bolt. As far as performance, I found that the XShear cut well, though not quite as effortlessly as the Raptor (my pair had some friction when opening and closing the blades). Still, this bombproof set of shears is the clear choice for a medical professional.
Leatherman Raptor - $80 (Response model) or $100 (Rescue model)
The Raptor comes in two configurations - a "Rescue" model with a seat belt cutter and carbide glass breaker, and a cheaper "Response" model that omits those tools. Both models fold down into a relatively compact (though heavy) package that can be carried easily, so if portability is important to you, the Raptor is hands-down the best choice; on the other hand, the folding mechanism is notoriously hard to clean. Cutting performance is on par with the XShear, though I found the Raptor easier to manipulate. Overall, this is the "Gucci" option for people who plan to carry their shears a lot but use them only a little.