10.5" owner here, always wanted a larger screen in 12.9" but deemed it too large in previous gen, checked out the new 12.9" yesterday and decided not to:
0) The 12.9" chassis indeed falls into hand-held or hand-carry friendly at last, especially without any cases or folio just by itself, it does feel just like a sturdy but light writing board that you can bring anywhere, but;
1) New pencil is nicer but not warranting an upgrade, it costs more and my old Pencil cannot work the new iPad... Magnetic attachment is neat and charging while doing it is great, but I question the practicalness of this setup as I swing around my iPad quite a lot, where I got a 3rd party case with a really secure Pencil holder. The new coupling method kind of limits how much I can abuse them while on the road without the Pencil flying out every time I take it out of the bag.
2) The smart KB folio is nice to type on, but pretty much sucks for every thing else. Very bulky when closed (is the back even necessary?), keys sticking outward when cover is flipped to the back for single-handheld which feels very strange for my fingers. Overall material feels cheap, and it does not offer extra fixing for the Pencil (in fact it may make it easier to get off)
3) Type-C is nice, but a lot of the functionalities were already possible previously with Lightning-to-USB3 dongles, with the exception of external type-C monitor and battery charging related stuff. Way too many accessories will need to be re-bought unless you are already invested in type-C stuff because of owning an MBP2016 onwards / rMB already. Having to carry an extra power cable if not an extra adaptor for the iPhone, and then also not being able to share the same Lightning earphones if you use one...
4) The Apple Store demo units purposely install very few apps, to hide the fact both the 11" and 12.9" will show black bars for majority of 3rd or even 1st party apps that have not been optimized for the new round corner interface. With the 10.5" even to this date there are apps that are not optimized and use a blown up interface with slight blurriness, I don't know how long it will take 3rd party devs for this round corner BS this time.
5) Cost. Extra investment in type-C peripherals as noted many times. Higher mark up for LTE than previous gen for no reason. Shady practice of only putting in 6GB RAM on the 1TB option, which asks for an absurd BTO price. AppleCare+ costs more now if I am not wrong. A totally decked out 12.9" + folio + Pencil + Care is running into a 16GB MBP pricing territory. How ever powerful the iPad may be it is still running iOS, it is hard to justify the price it asks with its limitations.
I will most-def sit this one out as my 10.5" is overall a solid enough experience without baby sitting the Pencil and purchasing new peripherals. The 12.9" is in a very ideal balance in size and weight and screen, with the 11" being not small enough to depart from that, and these alone are the only real benefits of this gen. I will wait when type-C gets more mature, particularly when an iPad attaching to a Mac can turn into an Astropad or something.