The formations in the Genestealer Cults codex provide the army with some very entertaining options for possible builds and come with some neat bonuses when employed.
Cult Insurrection Detachment.
For the price of a few formations, the entire army gets some very nice bonuses in this detachment. Patriarchs re-rolling traits if the warlord (and why wouldn't they be?); everything infiltrating (except vehicles); shrouded for those that already have infiltrate on turn one; manipulating reserve rolls by 1 in your favour and against the enemy; and regaining dead members that are in on-going reserves. This latter facet can be really powerful as it represents an easy way to get back a special weapon where needed, or even an icon. Overall, it is very powerful and can really be powerful when played right (particularly return to the shadows). Just be careful not to get wiped off the board whilst many are situated in reserves though -- that could really make a good game turn sour quickly.
Lords of the Cult doesn't do anything other than provide models. Meh.
The First Curse necessitates a large unit of genestealers, but they all get a bonus of some kind. Some are nice, others are situational or okay.
Brood Coven is okay and provides a number of vital special rules bonuses. This can be good for building a very powerful segment of the army that has an excellent attack possibility and even could approach a death star level of power. Almost.
Brood Cycle is a very large formation requiring a significant investment of cash to purchase the models involved. Having said that, it is nice as it provides bonuses to WS and Ld and a bubble of furious charge. It'll eat up some points, but resonates very well with the reserves back and forth that you will undertake with this army.
The Neophyte Cavalcade plays to a more mechanised army overall. It provides a way to grant outflank to vehicles and a 50-50 chance to ignore crew shaken and stunned.
The Doting Throng provides a nice bubble of zealotry and re-rolls to psychic tests for a Magus with this.
Subterranean Uprising is a nice way to grant infiltrate (and this in turn resonates with the Cult Insurrection rules, of course). The bonus for having the Primus is nice and not to be overlooked either.
Demolition Claw is another one that would be good for a mechanised approach. I personally don't like all the dynamite knocking around due to the short range myself, but I can see its appeal.
Deliverance Broodsurge is a good resonance with the Neophyte Cavalcade and provides good bonuses for taking lots of Goliaths. Really for the mechanised orientated force, but could work in other places as well (particularly high points value games). I would be tempted to stick to all infantry armies for the Insurrection though.
The other formations provide no particular bonuses, but are a nice way to gain purestrains (Shadow Skulkers), aberrants or metamorphs (Cult Mutants), and sentinels or leman russes (Brood Brothers). They're a nice way of tailoring the army list for particular themes.
My own take would be to go down a themed route: (1) all infantry and focus on the hybrids and everyone else; (2) mechanised - go all out with vehicles wherever possible; (3) pseudo-Imperial Guard and even take allies from said codex; (4) monsters and take allies from the Tyranids codex to represent something later in the invasion of a planet. All of these options (and more) are viable and can be represented through the Insurrection detachment readily.