I just received my Z2 last week, I've been keeping an eye on prices and they finally dropped to a level I was comfortable spending. I am coming from a G2 and shortly before that an M7 I guess here are some of my quick thoughts on it compared to what I've had.
Definitely a noticeable increase and weight and device size with no increase in size/resolution of the display. I had considered a G3 because of this but now after a week with my Z2 I've confirmed to myself that I prefer a premium handfeel even if it sacrifices usability. The phone looks and feels gorgeous. I also felt the iPhone 5 kind of felt cheap compared to the 4 and this reminds me of the 4 with a not as gorgeous edge. I picked out the purple one, and it did not disappoint compared how it looked online. OH I also love the notification LED, I haven't tested if it is full RGB but I love how large of an area it illuminates.
Speakers are not quite as loud as the M7, but definitely better than my G2, some of the factory Xperia notifications seemed just very quiet but I was happy with the volume after changing them to some of the other offerings.
I really like the UI compared to the G2, I am not big on flashing new roms. I am happy the light blue LG used for switches/sliders/highlights is gone. I am a big fan of the thin and light fonts Sony seem to use and how icons highlight to a white color instead of that blue (man I sound picky typing this out). Also seems like a lot less clutter in the UI.
Haven't compared the camera much, the preview seems much smoother and the photos seem better at first glance, I just picked up a compact SLR so really I haven't tried to use it for anything besides quick snaps to text to friends.
Overall for me I am glad I made the change I really like the build quality and U I, this is the type of phone I will actually put a nice case on and baby vs the G2 that I felt pretty indifferent about. I don't know if its something I can recommend to anyone else if I didn't know they were in to the whole premium feel thing because in the end it is bigger, heavier, and more expensive with no real benefits.
EDIT: Not sure if it matters but I am on ATT, service so far has been great after setting up the APN. I haven't quite gotten the speeds on LTE I've gotten on my iPhone 5 but that was almost 2 years ago and the network may just be more congested. I do like how the signal indicator for if you are on lte/3g/hspa+