[Xperia Z4 Tablet General] jelbo:
This might help people eyeing the Z4 Tablet, but are unsure of what positives and negatives there are. Of course, this is highly subjective, but this is my list. It's influenced by my personal competing choices which were the Samsung Galaxy Tab S2 and the Google Pixel C. I'm happy I chose the Sony Xperia Z4 Tablet.

Pros:

Fast SoC (Qualcomm Snapdragon 810)

This is Qualcomm's 2015 flagship SoC and from what I've experienced it's really fast. Android flies. It also runs 64-bit, which it should anyway, but for example Samsung's Tab S2 doesn't. I don't know about the graphical performance as I don't really play games.

'Compatible' SoC (Qualcomm Snapdragon 810)

This opens up the way for optimized-for-specific-SoC apps (like RSBrowser, which is Snapdragon-optimized and significantly faster than stock Chrome/Chromium) and CyanogenMod support, that need documentation/drivers. For example, Samsung's (faster) Exynos SoC's are a black box for developers, which makes things like this very hard and has the result of devs abandoning it.

Big internal storage (32GB)

32GB is plenty of storage for apps and a reasonable amount of media. But that can be stored on the microSD.

microSD capability (up to 128GB)

This is a major benefit for a media consumption device like this, which many devices don't have.

Good multitasking

I could have mentioned 3GB RAM, but that doesn't tell the whole story. Multitasking on the Z4 is pretty darn good. It swtiches quickly and is generally very snappy. My Samsung Galaxy S6 with 3GB RAM has pretty bad RAM management in comparison. I'm still trying to find a custom kernel for it that keeps the phone snappy after 2 days.

Huge screen solution, high ppi on a big screen

2560x1600, 299ppi. On a big 10.1 inch screen. This is wonderful.

16:10 aspect ratio screen

Which is good for widescreen content like movies and dSLR photo's. 16:10 also beats 16:9 for me because of the added screen height.

Screen has natural, accurate colors

Very subjective, but compared to several other screens I've found this one to be superior.

Front facing stereo speakers

A rare thing among Android devices. Good design choice.

Lightweight (~390gr), thin

It's pleasantly light to hold.

NFC, notification LED, GPS, vibration motor

These features are often overlooked, but are important to me. I use NFC for LastPass, the (multicolor!) LED with LightFlow to see what exactly is asking my attention when in standby, vibration to still be notified when I want the tablet to be silent and GPS for the occasional navigation need or social app check-in.

Qualcomm Quick Charge 2.0

Another nice bonus, which isn't mentioned much. Quick Charge makes a major difference to charging speed. Needs a compatible charger though.

Big battery (6000mAh)

Can't yet say battery life is amazing, because I'm using it a lot and crank the screen brightness up quite high so don't know what to expect. Reviewers seem to agree it's great though.

Bootloader can be unlocked (so the road is open for rooting)

No waiting for an exploit if you're OK with going this route. Just follow Sony's instructions and you'll have root in no time.

Marshmallow announced

Should come January '16 I heard, but these things always get delayed :| At least it's coming.

AOSP commitment by Sony

Sony's Open Device Program is nice and all, but their sources are a bit troublesome and don't seem to produce functional ROMs. Still, Sony's stance on it might bode well for future things.

Water-/dustproof

I don't care much myself, but it's a nice bonus. At least it takes some worries away (dropping liquids on it, no fear for dust particles between the screen and the glass).

Keyboard dock option

Nice for when you want to use a physical keyboard that is fully compatible and is also attachable. I use a 3rd party BT keyboard, but I'm constantly fighting with fixing incompatible button mapping stuff.

Important root-specific things that work

These things are not guaranteed to work or be available on any rooted device, and are pretty major in adding possibilities, so I consider them pros to be working on the Z4T:

Xposed Framework

For most people anyway (Some are having issues). This is a thing to be happy about, because if it didn't, chances are it wouldn't be fixed anytime soon because of the small user/dev base. Xposed opens up many possibilities which really enhance a device. To me it's a selling point.

Native KCAL support

Another Qualcomm exclusive. I believe this is actually fully present on the stock ROM, but not fully controllable (limited to RGB in the Settings menu). KCAL support enables you to tweak various image parameters, like RGB, saturation and contrast with a tool like Color Control or Kernel Adiutor. It's pretty great and you don't see it often.

Cons:

SoC might overheat in extreme circumstances

Haven't had any problems myself, and I stress the tablet pretty hard, but I've read some reports about issues. At least of a guy bringing the tablet to the beach. It's mostly just people saying it's fine, even with heavy usage.

Speakers are lacking in bass

No surprise, but it's still a letdown.

Bad low-light camera performance, no flash

Picture quality in low light is disturbingly bad. Having no flash makes this unusable in those situations. Not a big deal for me personally, I don't take pics with a tablet.

Screen isn't that bright

Compared to several others, the screen isn't that bright and needs to be cranked up pretty much, even indoors. Outdoors, this is a problem. The big screen reflectiveness doesn't help either. Indoors it fine, it just that the needed high brightness level eats battery.

Screen lacks deep blacks

This is compared to (S)AMOLED, specifically. Those screen blacks are amazing and darker colors are also good for battery on those screens. IPS screens just don't have that. Using dark themes won't help battery life on the Z4T, it may even be worse with them.

Stock charger isn't Quick Charge 2.0

Come on, Sony.

No hardware navigation buttons

This is a real PITA for me because this requires Android's soft keys / navigation bar which take up valuable screen space. This is especially problematic in landscape mode on this 16:10 ratio in which you'll want every screen height you can get. Fortunately, this can be overcome by tools like GMD Full Screen Immersive Mode (with full screen keyboard typing restrictions so you'll have to switch back to type :S) combined with All in One Gestures, both of which don't reqquire root. Better yet is a build.prop edit that declares to Android the tablet has hardware buttons, removing the soft keys entirely, while keeping the ability to type anywhere. I navigate using All in One Gestures, because GMD GestureControl sometimes stops working. Which isn't very nice when you don't have navigation keys :P

No user-land root exploit (yet)

Because of this, you'll need to unlock the bootloader to gain root access. Which will destroy your TA partition, which will in turn remove Sony-proprietary functions. Which I personally don't use and don't see much use for anyway. Also, unlocked bootloader can't be undone without Sony noticing, so as a non-EU citizen you'll possibly have warranty issues.

Small user/dev community

Not many people own a Z4 Tablet (bad availability in the US and it's expensive) and because of this, there's next to no development for it. Luckily, we have [MENTION=5169444]AndroPlus[/MENTION] who's made a custom kernel and ported TWRP (which unfortunately has a bug that keeps us from restoring the system partition from a backup). [MENTION=6774113]DHGE[/MENTION] worked on root, which made it possible in the end I think. Still, custom ROMs would be nice. Also, if you run into device-specific problems, there's not many others that can help, because you're either the only one or one of very few who have that problem.

It's expensive

The price is very high and a bit hard to justify.

What I miss:

Wireless charging

This is sooo convenient. It also spares the precious MicroUSB port, which is used for charging, data-transfer, USB-OTG and adb/fastboot. If it breaks, you're done.

Removable battery

Batteries do not have eternal life, so eventually it will be completely dead. Which will render the tablet dead as well.

Any thoughts, questions, additions or critique is welcome.