So got my first match against Necrons tonight. Wes brought an unusual Necron list, with lots of Immortals in no transports, Stormlord, 2 Monoliths, 2 C'tan Shards, 2 Destroyer squads and some dude with a Harp of Dissonance (I think?) who did nothing other than die.
So, the old tactics for Necrons don't really work. Immortals should take advantage of open-topped transports if possible, because they bring some good dakka. 3+ armor is pretty good, I hear.
C'tan Shards will either need help getting across the board or will have to remain behind as a counter-attack unit. Cool as they are, I never felt particularly threatened.
Monolith's Portal to Exile went off 3 times, claiming only 1 Fiend of Slaanesh (that had only 1 wound left anyhow). It's not as scary, especially as the Monolith can be flanked, forcing the Portal to one or the other, but never both.
Personally, I'm in serious like with the Triarch Stalkers. I strongly believe these units are heavily under-rated. It shoots at a target, which then grants twin-linked to everything else shooting at that target. Perfect anti-Death Star utility!
Initiative 2 is a real bitch.
I think this codex really has all the tools to succeed. It can certainly customize lists well enough against any army.