I am not calling this a tactics article, because I havent played any games with the Cryptek yet, however it is an article based on the Theory of them.
So for a decent price you can pick up a Cryptek, or more specifically 5 per Overlord (or equivilant you have as a HQ).
I see a lot of people using them as the Harbinger of Destruction, mainly for the Solar pulse and some Str8 AP2 shooting.
I have also seen a few people using them as Harbingers of Despair for the Veil of Darkness again a valid option with provies some useful jumping around.
Personnally I will be running a couple of the Harbingers of Transmogrification. Their Tremorstave is execellent. A 36" range weapon that makes any unit it hits count as if they are in Difficult Terrain. Note it states Hits not Wounds so you can target a Landraider with it and as long as the template doesnt scatter too far you will be ok. An immobilised tank is a tasty target for Scarabs.
The ability to slow down assault troops is also an excellent choice as it can really screw up someone's charging, and that extra phase of shooting can never be under estimated. Just my opinion, but we will see how it plays on the table top!