I have fully embraced KT21, took Necrons to Adepticon and had a blast.
The mechanics are different and less recognizable as 40K; no WS BS etc. It is action points with moving, shooting and fighting. It is still single model alternate activation.
The old teams may not be valid. My old Necrons had individuals like 3 Immortals, 2 Deathmarks and 2 Flayed Ones. There are no points so you pick two "Fireteams" from: 4 Immortals or 4 Deathmarks or 5 Flayed Ones or 5 Warriors. This is restrictive but no more plasma spam, etc.
The Kill Team boxed "armies" are a bit more powerful than the Compendium "get you by" teams and boxes teams generally have a medic, fighter, demo, comms, heavy of a sort.
The release Octarius box is the best "value" otherwise get the rules, essentials, and an army or Killzone. Check out Wahapedia for rules and army lists.