I also came from the S3 and agree regarding the notification LED and the soft-keys. The LED on the S3 was perfect (the Z, too), however the Z1's is definitely harder to see. I solved the soft-key issue by switching on pie controls in xposed/gravity box and deactivating the Android soft keys. I can easily use the phone single-handed, tapping the pie keys with the thumb.
I quickly got used to the less explosive colors (which imho are far too saturated on the S3/4) and enjoy the better handling in bright sunlight compared to the S3.
My biggest con is the phone's size. It's so large I don't want to put it into a flip-cover, which would make it even bigger. On the other hand the naked phone is a bit difficult to hold because it's quite slippery.
I've had the black one for months now, pretty good,