A variation on “how many pipers are there in the world?”. Best guesses:
What proportion of pipers play flat sets exclusively?
What proportion play both flat and concert sets?
What proprtion play only concert sets?
I guess I’m just a curious guy. 
I think you can set this up to where folks can “vote” to indicate their answers. I don’t know how to do it but maybe someone else could help out?
No, not with these sorts of questions, which call for numeric answers.
But I’ll remove “POLL” from the thread title, since people here will think that implies one of our standard multiway polls.
- Mod
I have two concert sets and two flat sets. The concert sets I play in public and the flat sets are played at home. Flat sessions are precious few 
To answer your question, I play both.
But becoming more common than they used to be. Last year at the Northeast Tionól (north of New York City in the Catskills) I remember there was a large C session as well as a gorgeous all-night-long B session (I don’t think I’d ever encountered a B session before). C pipes are very common these days, and you can find weekly C sessions in some cities (I heard there was one in Toronto, though I’m not sure it’s still happening).
Lesl and I are already sorting out C-flute timesharing for this year 
A good few pipers will have a concert pitch set and a few flat chanter (C, B, etc.).
All of the pipers in my household do - ok, there’s only me that plays but that’s statistics for you.
Jon
Bear in mind also that a good few pipers these days who tend to do mostly solo playing might have just a flat set and then a concert-pitch chanter for when they’re playing in gigs and sessions with instruments that can’t tune down.
i’ve got some idea why flat pitch sets are desirable but i don’t understand on what basis you would choose one flat pitch set as opposed to any other
I’ve heard more than one piper say they chose C# either because Ennis played in that pitch or because they thought it was the “old” concert pitch.
Jerry O’Sullivan mentions on his website: “In terms of pitch, I had opted for a C set as I feel that this pitch has the sweetness of a flat set but still retains some of the punch of a D set.”
Personally, I like the sound of Bb.
There are other reasons too: not being able to spread fingers wide enough to cover holes on a B or Bb chanter, or not being able to reach certain regulator keys.
I’ve long suspected that they look around at what keys of whistles and flutes the people around them own, then pick a flat pitch that’s different. Keeps the riff-raff from horning in on their chunes. 
I really think it’s the sound. There’s a big difference in sound between concert-pitch sets and flat sets, but there’s also a big difference between C sets and B sets. And there’s a big difference between the different types of chanters in the same key – some C sets being mellow and quiet (and not conducive to tight piping in the first octave) and others being big and loud and perfectly capable of handling tight work in the first octave without jumping.
The lower-pitched chanters either grab you or you don’t (listening to Ronan Browne’s monster low G chanter always makes me think of a flatulent elephant); I loved the sound of nearly every C set I’ve heard but find a wider range of timbre in the flatter sets like B and Bb, some of which I love and some of which I find hard to listen to. It’s a personal thing, I guess.
I have a C half set and a D chanter for lessons. I prefer the sound of the C set and it doesn’t bother the neighbors.
Does anyone play a C chanter against a set of concert D drones? I’ve heard that creates an unusual but not unpleasant effect. I’m currently learning on a practice set with both a concert D and C chanter and trying to decide which way to go with drones in the future. Don’t think that two sets is an option.
I’ve tried it but even with a relatively loud C chanter, the D drones drown it out.
If you want to hear what it sounds like, have a listen to Liam O’Flynn playing the Humours of Barrack Street (about 30 seconds into Timedance).
I play in D at sessions around here, and opted for C as a flat pitch because that’s what my slightly-more-distant friends who play flat sets have. Also the stretch on some B sets is tough for me.
C chanter against D drones would be really nice for D minor tunes, or for playing what would be E minor tunes on a C set. But otherwise…? C drones are so wonderful, why else would you turn them off?
KAD
I’m probably not the only one on here who put that record on the turntable, listened to the first few seconds and thought, “Wow, I didn’t know they made pipes pitched THAT low”, and then realized that it was a 45 rpm EP…
You better explain the terms “record”, “turntable” and “45 rpm” for the benefit of the younger forum members … 
As long as were on the topic- I was wondering if it were possible to play C regs with a D chanter. The reason I ask is that on Ronan Oliviers website, he makes C/D convertible drones in which if you order say a 3/4 C set with convertible drones and have both C and D chanters you have in essence a 3/4 C set and 1/2 D set. Anyway, if your playing it as a 1/2 D set, would the C regs be useless or do you have any note/chord possibilites to use on occasion?