Some observations
1. Obviously it is so much more convenient than North terminal. The shuttle run is a thing of the past!
Nice for those who use the Train. Not everyone does.
2. The check in zone is inconveniently at the far end of the terminal, and then you have to walk back the same distance to security. Norwegian has prime spot but I guess they were there first.
Closer for those arriving by Next coach from LHR, = 50/50
3. Premium security was a dream, and the staff were so polite. But I've always found LGW security to be far far superior to LHR.
Agree 100%
4. The lounge is now clearly signed immediately after security but you still have to walk through the upper shops level. And boy is this terminal crowded! If you don't have lounge access it would be an unpleasant wait I think.
FLounge nice. Terminal awful!
5. The lounge seems to be down a warren of corridors and lifts, so I was a bit surprised to see that so many other people had found their way to it! I found an empty set of seats upstairs but that was after searching for a while.
Secrets .. it will keep the occupancy low 
6. The lounge is nice (it is new after all) but why oh why are the gents bathrooms so small? There must have been 100-150 men in there and 2 cubicles. The queue was out the door. Crazy given they had so much space to play with.
Only used the FLounge ones, which were OK ... stupid design using cubicles for chaps, we don't take that long..
7. The walk from the lounge to gates 25-28 is seriously far. I'd almost say it's worse than the bridge over to the remote gates at north terminal. And you have to navigate your way past the elderly and the bucket-and-spaders.
Long walk next month, but some elderly can move quite quickly. Wanna race?
8. I walked fast as the lounge screen had said hey flight was boarding. Not happy to arrive and see it hadn't even begun.
And the Lounge screens only show BA flights, which seems a bit unkind.