Tone: Mostly pure, with a slight woodsy chiff. Unlike other large-windway whistles (like say, the Clarke original), you don't get the big whooshy windy sound. The sound is crisp, clear, with just the right amount of complexity.
Volume: Loud. LOUD. Your deaf aunt can probably hear this whistle just fine. Seriously, this is a loud whistle. At my loud session, I used to bring out my Oz whistle when I needed to lead a tune and nobody could hear even the Abell. But this whistle wins the prize. I've played it exclusively at the last few sessions I've been to, because they've been banjo and concertina heavy. And it stood up to them just fine. If I still worked at the Renaissance Festival, this is the whistle I'd be playing outdoors to help the tune carry.
Responsiveness: Quick.This whistle is just as quick as my Abell, Oz, or Copeland. I have no issues at all playing whatever ornaments I want without muddiness.
Tuning: When tuned to A=440, this whistle is in tune up the scale, with gradually increasing breath. No special adjustments necessary for unruly notes. That said, when I blow the second octave as hard as I think I need to, I tend to be about 15 cents sharp, so I'm going to have to learn that (unlike all of my other loud instruments) the second octave takes a lighter touch.
C-natural: OXXOOO makes a perfect C-Natural. All of the other alternative fingerings I know leave the whistle extremely flat (30-40 cents).
Hole size and placement: The holes are centered on the whistle and fairly evenly spaced--though, like with most whistles, there's a small gap between F# and G, and the E and F# holes are a little closer together. But unlike most whistles I own, the holes are all very close to the same size. The E hole on my Thornton is considerably smaller than the E hole on my Abell. This is good news for people with small fingers who might have trouble sealing a standard whistle.
Air volume requirements: High. Like Clarke Original high. As a comparison, I grabbed my Clarke out of the whistle drawer and played them back to to back. I ran out of air on the Clarke first, but I only managed to get 2 or 3 more notes out of the Thornton on the same amount of breath. This would have been a deal-breaker for me at one time, but I'm at a place in my playing now where I'm a lot more comfortable finding new places to breathe. It might put beginners off of the instrument, though.
Air pressure requirements: Medium, tending to high. The whistle doesn't take nearly as much pressure as I would've expected, and if you really lean into the second octave, you're going to blow way sharp (as I note above in the Tuning section).
Clogging: None whatsoever. But that's not really that surprising with a whistling moving that much air through such a large windway.
Wind Resistance: Good. I haven't really had the opportunity to play this in really gusty winds yet, but I haven't had any problems playing it outdoors. Some whistles die with the slightest breeze. This one seems fine for ordinary weather conditions.