Just started this topic in order to give my opinions about this recorder.
@Hexacore300 showed interest in H6 in a thread related to another recorder, so I prefer to write here.
I just received a Zoom H6 and I have been playing around with this comparing both internal capsules ( XY and M-S), also comparing them to three mics through XLR. I've used both internal preamps only and then plugged my external Naiant Littlebox preamp into XLRs in order to minimize the internal preamps in favor of the supposedly better in Littlebox. Just compared all this to my old Edirol R09HR with the same Littlebox ( different connection, though).
Here is what I've found after playing the short clips on Audacity normalized at -2 db ( no more than a short portion at the piano at 70 cm distance):
The good:
1) Both capsules perform admirably well for the price. The M-S capsule has been commented as being noisy: probably this issue-if ever- has been fixed but I didn't find it noisier than the X-Y, which isn't noise at all. It's simply 3 db more sensitive than X-Y ( -38 vs -41db), so you have to keep the level a bit lower in order to match the levels.
2) I used three external condenser through xlr, being 2 of them much less sensitive than the stock mics ( -48 and -50) and with a not very good self noise ( 21 and 25 db); the third being more sensitive (-32 db, 21 b self noise). In the need of lots of gain the support of the external preamp produced a slightly better performance than the internal preamp, but with more sensitive mics or closed miking I'd say the performance is on par ( plus, none of my mics is low noise, which is the main problem mostly if coming with a low sensitivity spec). In general the stock internal preamps offer a decent performance and I can't here any bothering hiss over the self noise of the mic.
3) The overall result, compared with my Edirol R09HR, let me perceive a more dynamic range on the Zoom recordings.
4) H6 is very versatile, as may be plugged into a computer and become an audio interface very easily. You can power it also by simply connecting a little and cheap battery pack like the ones you use for cellphones ( 5v, 1A)
The less good:
1) the headphone output is not that great. It can drive my 150 ohm Sennheiser headphones at an adequate volume for monitoring or hearing back what's been recorded, but I wouldn't call it a real hi level listening experience. I also noticed a slight background hiss at high volume which made me suspect about preamp noise, but nothing of this was present in the recording, so it's just not a great headphone performance. H6 has also a line out socket, which can be set at various levels below 0 db; I just tested plugging my headphones- which is not supposed to be done-. As I could expect it sounded maybe cleaner but a very low volume (it's a line out), so I can't really tell if there is any difference in quality.
2) Not really an issue, but something to be aware of: sort of a learning curve is needed in order to properly match levels. Little dial movements (i.e. from 5 to 5 and half) produce appreciable differences in level.
Overall I'd say the H6 is packed with features and, while not being a professional grade piece of gear it is capable of doing a lot of things quite well. The changeable capsule are a plus as they are an all-in-one solution and will be adequate for a lot of situation. Just a buying advice; if still available, buy the "old" grey version instead of the "new" black one. The new version is exactly the same as the old, it will cost you more money and it will be shipped without the m-s capsule and without the carrying bag.