I was there on the 9th for a 5 hour transit when it was first operational and they were handing out promotional "T3 caps". I thought it was rather funny so I took a couple (cap for airport terminals...) Quite impressed that given it's the first day, all the shops and facilities seems to be ready - only a couple of "coming soon" shops. Having said that I noticed many shops' staff were discussing the layout and how to improve it.
Overall impression is a nice terminal with a lot more (and better) seating and lots of free Internet terminals than T2 which is good if you don't have lounge access. E.g. they have reclined seats (like a beach chair) along the tarmac sides of the terminal. Similar selection of shops to T2.
The only negative thing is the actual boarding gates. Unlike T2 where most gates have their own screening area, I boarded at gate A5 which had to share its with a number of other gates. I waited till last minute to board as the screening area is very close to the gates. It turned out everyone had the same thoughts and the screening area was totally chaotic. Definitely prefer T2 on that aspect.