Just received a Chieftain Low D from Phil Hardy. This is one of the redesigned Chieftains and the differences are immediately noticeable. I owned a Chieftain Low G purchased a year ago or so, and although I liked its sound, I found it a challenge to play because of its high air/backpressure requirements and some clogging issues.
This Low D is much easier to handle. Its air and backpressure requirements are considerably more reasonable (not as low as an Alba, say, but in the general range of an Overton. I know, I know, please let’s not start that one again!..). No clogging issues so far, and I am a bit of a drooler when I play.
I am not an expert, but it looks like the head was somewhat redesigned, with softer contours. Perhaps Phil can elaborate on the technical details?
I really like its sound, too. I find it different from my Low G in that category as well. The old Low G had the volume of a foghorn. This Low D is much more mellow, with just a bit of chiff mixed in. Certainly not as quiet as a Dixon, but it won’t make people jump out of their seats in fright.
My only (small) difficulty with it is a rather large 5th hole (that would be 5th starting from the top). You have to be careful to cover it properly and it definitely cannot be handled without piper’s grip. But this is a minor thing and just requires a bit of adjustment. The finger spread in general is not too taxing (I do have fairly large hands).
So, all in all, a whistle well worth considering if you are in the market for a Low D.
PT
I’ve had one for about a month now, and I think it’s great. It plays great and is very easy to handle. I’m a beginner and the only other low whistle I’ve played is a Howard that I’ve had about two months. The Howard is frustrating for me to play because of the reach and huge holes (although the thinner walled brass is more “tactile”). I have some difficulty with the second octave on my Chieftain “Gold” soprano D, but not so with the “new range” low D, and it certainly warms up a lot faster.
As a beginner I would recommend it to other beginners, and it doesn’t cost much more than the Howard. (I don’t mean to knock the Howard, it has a very nice tone, but my skill is not up to playing it.)
I am also waiting for an Overton Low D. Many here seem to like it better than Chieftain, and if it really is better than it must be truly amazing to surpass the Chieftain.( I know, I know … I should learn to play before buying all these whistles! 
)
I have about the same as Patrick, except it’s not tunable and in low Eb.
I agree with all Patrick says, and I consider it as good an aluminium low whistle as can be. Good low end, goes up to Eb" easily enough, well-balanced. Low wind requirements but moderate resistance. A bit of breathiness I personnaly like.
Also I personnally like the large 5th hole, enabling me to play or slur clean F-nats even in low register. My Copeland low D btw also features such an oversize 5th hole.
The new releases are quite different from the older Chieftains I tried (and didn’t buy), and it almots soothes the regrets about my rare Kerry Pro low Eb, which I moosily abandoned to a non-Tibetan llama knniggettine. 
Patrick, one of the obvious differences with the older Chieftains is inside the window. A bit like recent Albas, it has “cheeks” inserts, which have the same function as the square shape of Overton-style heads, hammered flat each side of the window.
Also, the aluminium seems of a higher quality (drawn tubing) which should give a smoother bore and maybe improve resonance, too. Drawn tubing vs extruded is like forging compared to casting.
Zoob, thanks for the technical details. It does look and feel like high quality aluminium and the instrument is quite a “looker”.
The large 5th hole seems to allow the lower three holes to have more even spacing, rather than having 4 and 5 close, with 6 more distant.
It definitely makes the hand position more comfortable on larger whistles. I’m pretty sure that if my bass-Bb didn’t use that technique, there’s no way I could play it the way I do. (fingertips)