I read some rumors about a Z4 being shown off behind closed doors at CES, along with the Galaxy S6 and something else. I hope that they are just rumors for the Sony, since the Z3 just came out 3 months ago, and I just got my black D6603 2 weeks ago. No need for a replacement unless Sony wants to alienate those who just bought one.
(If I bought a Galaxy S5 or One M8 in Spring 2014 at launch, and they released the S6 / M9 in Fall 2014, I'd be a little pissed off that they retired my recent phone so soon after it's introduction.)
As for my phone, here are my impressions. (having a Nexus 7-2013 and a Nexus 4 as reference)
-It has speedy performance in most apps, only my WeatherBug app takes time to load.
-Camera is good, though coming from a Nexus isn't giving much of a comparison. I like the camera button, having missed it from my old myTouch 4G.
-Speakers are pretty good. Can distort a bit at max volume, but go one step down and it sounds nice.
-Sony specific apps want to update independently from Play apps. I already have a suite of apps I used on the N4 that I also use on the Z3, so I don't bother with Sony's media apps and such.
-The basic file manager that's on the phone lets me transfer files from the SD card to the phone memory, unlike ES File Explorer which does not have permission to write to the SD card. (I may root the phone just for that purpose.)
-I am not a fan of having to manipulate my hand to reach the upper left corner of the screen (why do some apps make you hit that area of the Play apps to access the side menu?)
-The option to double-tap the screen to show the lock screen is nice.
-The magnetic charger is interesting. The two 9 dollar charging dongles I bought are a little fiddly to line up just right, but plugging in a microUSB jack can be fiddly with some cables as well.
-With the side ports and buttons needing access, case makers have their cases extend along the sides of the glass, making swiping from the side of the screen to access some menus (or drag an icon from one home screen to another) a pain in the ****. I would love a thin hard case that just came up over the corners like my N4's Ringke Slim.
-Copper would have been awesome, was out of stock when I bought mine. I'm thinking of getting a wrap for the back glass if I can find one with the SONY logo cut out.
Hope some of this helps.