Tone: This whistle is fairly pure, but with some small amount of overtones that give the instrument a nice, round tone. It sounds pleasant and full, and not piercing like a completely pure whistle can. The Shush modifications take out those harsh overtones that can be found in a Feadog that sound grating to me after a time.
Volume: The Shush lives up to its name. It's very quiet. The sound meter on my phone (which is not a precision scientific instrument, mind you) calls the 1st octave A 71.3db. My McManus whistle, at the same distance and breath from the mic is 80db. That's a big difference!
Responsiveness: Very nimble and responsive. No issues with muddled-sounding ornamentation even at my fastest speeds.
Tuning: This whistle can be blown into tune, but will take a little getting used to. The D-E-F notes require very little push. When you push this whistle harder, it does get louder but also up to 20 cents sharper. Things change at the G-A-B notes, where you have to push a little to keep the whistle from being flat. Then, in the 2nd octave, the D again comes down in breath requirements. It's not too much gymnastics, and learning a particular whistles breath requirements is just part of playing these kinds of instruments. More concerning would be if there was an odd note in the middle of the bunch that had drastically different requirements than its neighbors--and this whistle doesn't suffer from that particular flaw. Note that this whistle is tunable. I thought it wasn't at first, because the head fits rather tightly on the body, but after confirmation on the shush website, I got a bit more brave, and with a little more strength, the head came right off and was able to be moved along the body for tuning.
C-natural: OXXOOO C-natural is spot-on. OXXXOX stabilizes the note just a smidge, and OXOOOO is nearly 40 cents sharp.
Hole size and placement: The holes are centered, and well-rounded. The whistle shares the configuration of many others, moving the E hole closer to the F# hole, which allows for it to be a little smaller and more in line with the other hole sizes along the body. But it's not so close that my fingers rub together when playing, which I've experienced on a few other whistles.
Air volume requirements: Low. I have a few tunes where it's a challenge to find breathing spots for on air-hungry whistles. On this one, I don't have any problems.
Air pressure requirements: Low. This is just an easy whistle to play, all the way up both octaves. If you're used to more aggressive instruments that take a lot of push, this might take some getting used to or you'll blow the whistle out of tune.
Clogging: Like the Feadog it's based on, this whistle is middle-of-the-road when it comes to clogging. I was able to play it for a good half-hour before I had to blow it out, and I imagine that duponol treatment would extend that playing time considerably.