I got my Serpent Low F yesterday. Didn’t post this last night because I was too busy playing it 
I’ll post further impressions in a few days, once I’ve had a chance to play it more.
First impressions:
Physical:
Lovely whistle - I ordered the sating finish with serpent (what else?) engravings. Simply beautiful, and a very easy reach. Very stable tuning slide (and in-tune with our piano at the marked position - a plus). Slightly flattened on the bottom of the tube, opposite the line of finger holes. Since the finish isn’t marred and the shipping tube was unhurt, I’m guessing this is deliberate, to prevent the whistle from rolling. (VERY cautious experimentation verified this works). Massive - not at all uncomfortable to play, but this is a heavy whistle - it’d do well in a bar-room brawl 
except for the fear of marring the finish. I could barely feel my high D whistles, or even my Generation Bb, after playing this one for a while.
Playability:
Medium quiet; fuller than my Dixon Low D at the bottom end, better balanced on the top, lovely throughout - the second octave “B” (the one-finger-note, actually “D”) isn’t at all shrill and still has that pure-with-a-hint-of-chiff tone. Seems to take a lot of air, though less than I first felt. Certainly more than my Dixon Low D, but that’s a very low air whistle.
Still, I can tell that I’ll be working on my breathing a bit - I keep running out of air at odd places on some tunes. Even so, it did beautifully on a selection of slow airs, a few marches, “The Star Spangled Banner” and even a few jigs and reels. (I was particularly taken by “Smash the Windows” and “Rolling in the Ryegrass”).
As I mentioned under “Physical”, this has a very easy reach. I don’t have huge hands (broad palms, but short thin fingers) and I didn’t even have to think about the fingering - it just fit in easily; I had a much harder time going from high D to my Generation Bb the first time I played it. There’s something about the hole spacing that really works, at least for me - I find the Sweetone/Meg C layout harder to adjust to.
Thanks, Bill. I love this whistle.