Termagants are cheap, at 5 points a model a sizeable unit can weigh in at around 100pts. That said they are also one of the worst units in the game (in their basic form). No real armour, a stat line that defines the average (in an imperial guard barracks anyway) and a bolt pistol. Thrown in their lacklustre leadership and special rule that means they'll sit and drool unless watched over by a synapse and they really aren't anything to write home about.
The key there though is 'watched over by a synapse'. Once within 12" they gain fearless and their main role in my force comes out. They are an excellent charge blocker, able to soak up a good 8 wounds before having to worry and sure they aren't likely to cause much damage back (more on this later) but they'll stick around and hold the opponent in close combat for a devastating counter charge. As units locked in close combat don't gain benefits of cover when subsequently assaulted this can be a useful way of shielding a valuable unit and then 'revealing it' as the termagants start to die ready for a counter charge.
But who says termagants can't cause damage? The real strength of the termagants is their ability to allow a tervigon to slip into the troop choice. Suddenly all of a termagants drawbacks melt away whilst this MC sits within 6". With counterattack and toxin sacs the termagant can suddenly hope to not only tar pit but cause some damage to an assaulting unit, just make sure its within 6" AFTER combat moves.
To guarantee this deploy the termagant blob (about 12-15 models) where you want them. Then form a conga coherency line back and even behind the tervigon. If the unit is assaulted and pulled forward 6" then the rear elements of the line should still be within the crucial 6" to confer the goodies to the tervigon. Looks like the tervigon might go down this shooting phase? If possible remove the conga line and cut the 6" link to save some models for when it finally bites it.
Able to generate troops choices and sit happily outside of combat feeding I5, Str4 poisoned termagants into an ongoing fight makes the termagant/tervigon relationship one of the standout features of the current Nids dex. It won't suit every force and due to the points cost will define a great deal about how the army will function but for the commander after a scuttling swarm Nids army, its perfect.
That said a danger is relying too heavily on the trick. Termagants and tervigons are ways of turning a poor unit into a mediocre unit but one highly dependent on a model that can be brought down in a single turn of focused fire. More of a good thing is not necessarily better. 100 rubbish troops on the field isn't going to get you much further than 60 rubbish troops and their role as a delay and block doesn't work if you have nothing hard hitting to take advantage of the gap.