Hi folks,
Just picking up this old thread to introduce myself and have a go at the topic.
I have bought the Dixon one-piece a few months ago. I know of only one place in the Netherlands that sell Irish flutes (http://www.celticshop.nl/), so that’s where I went. (If you know of other places around, then please let me know!) Both the one-piece and the three-piece look like they are mold cast and of similar processing quality, with the three-piece being six times the price of the one-piece – and equal in price to a lot of handmade and appraised mid-range instruments unfortunately not available in the store.
So it was a no-brainer, really: I got the cheap one-piece to get a taste of the ‘simple system’. To my delight I found that I can play this keyless D instrument together with a G/C diatonic harmonica on more occasions than I would have thought, without having to fall back to my Boehm. After having played it for a while, these are my observations:
First off, the bore is dead straight. If the instrument is really cast, then it wouldn’t have hurt to make it conical, now would it? As for it being audible, I think you require the upper part of the second register to tell the difference (right?), and I must say I don’t get there often enough to bother (for now). Sometimes after extensive playing I will find that the flute is slightly off with the harmonica, but apart from that, it seems that attention has been paid to the overall intonation, which turns out quite acceptable.
Second, the embouchure hole is a monster. It is even bigger than my Boehm’s, and bends noticeably with the instrument’s curve. You have to actually blow ‘into’ the hole quite a lot to make the lower notes sound anything expressive, and then roll out again for the higher notes. Maybe that’s a trick to enforce automatic correction by ‘lipping’ up/ down?
What frustrated me the other day, was that my band mates asked me to opt for my Boehm during practice, so that I would be audible over the harmonica. Apparently tI am not that loud! (At home, I like a mute flute though, because my neighbours hear everything and I can’t refrain myself from playing in the middle of the night.) By experimenting with making PVC flutes, I am used to many different embouchures, so I don’t believe it is because I have to get used to this flute. Maybe this is a property of the flute not being conical in bore? I must say that it is still a lot louder than my PVC Low D/C experiments using less-than-optimal pipe diameters, though. (Doug Tipple, if you’re reading: I just can’t figure out how you would get a reliable low D out of 3/4" PVC – internal diameter only ~1.6cm!)
As for the ‘next step up’, I am still in doubt. I should expect having to play in C, F and G often, which would either call for a C flute (love Billy Miller’s bamboos from what I can observe; wish I lived in such a climate!), or at least a key for playing natural F (can’t cross finger that one, right?). I just love the fully-keyed instruments, but they either require taking your hobby very seriously, or thinking that money is a joke 
(I also don’t feel any good about mail-ordering a quality instrument just from sight.) In the mean time, there is still a lot of practicing I can do on the Dixon, though. Gotta get my fingers up to speed!