Everyone facing the new Tyranids will probably be seeing more Termagants. Not because they are good or really cheap but since they allow you to take a Tervigon as a troop who in turn spawns more Termagants.
Now the basic Termagant with fleshborer is a point cheaper but he has lost fleet and the living ammunition rule while gaining a point of leadership. The living ammunition rule itself made him a 50% better shooter so the lose was probably not worth the point. He is pretty much now a guardsman with a bolt pistol and +1 initiative but worse save.
They have a couple of options for load out with complete changes to spinefists, spike rifles, or devourers.
Spinefist exchange a point of S for twinlinking. This is a wash vs T4 targets, and a 12% increase in wounds vs T3 but since the unit costs 20% more each now and they have no hope vs even the lightest vehicles it is a bad upgrade. The only time it is worth the points is against T6 opponents where the spines are 25% better(due to the repeat 6+ to wound on the tables). So spinefists are the way to go verses nidzilla. In general though, they should be costed the same as fleshborers such that you are tuning the unit for low or high toughness targets. If you see spinfists on the board it is because the opponent already had them modeled and did not want to change them.
Spike Rifles trade 1 point of strength for 6 inches more range. Also increase model cost by 20%. This is much more of a tactical based decision but against T4 that point of strength reduces your shooting quality by 33%. Might be worthwhile trade since 6 extra inches range could mean a whole extra turn of shooting. But even if you double your shooting for the whole game(not easy) you are essentially just get 33% more wounds compared to the same number of fleshborers factor in the model but you have 20% fewer shooters. Factoring that in I think this is another bad upgrade. Should be even cost.
Devourers give you both 6 more inches of range plus 2 additional shots at the loss of the AP value. This upgrade double the model cost so that significant increase in firepower is essentially mostly canceled by having half the models. So on a fixed cost basis you will be generating 50% more wounds but also have half the wounds to give for a unit with a very bad save. The 18 inch range helps with Lurking and if you could put the unit in cover that was not going to be flamed or assaulted it might work out. I think this upgrade would have been better costed about 3 points per model.
Now for every 10 termagants you can get a strangleweb. It is an interesting concept: Wound vs Strength instead of toughness with a assault 1 pinning template. Problem is obvious though in that is S2 weapon meaning 5's to wound the weakest normal opponents and 6's for standard opponents. It is also Ap - which means everyone gets their armor saves. I have a hard time expecting this to generate any unsaved wounds to force the pinning check. You need 18 marines under the template to expect a unsaved wound. Only need about 4 orks but they generally are not worried about pinning when the are together enough to get great template coverage. Had this weapon had a Strength of at least 3 it might have been worth while but I think to cover its listed cost of 2 termagants it probably would need to be S4.
There are also 2 biomorph options: Adrenal Glands which grant furious charge and Toxin Sacs which give a Poison 4+ to the close combat attacks. Each of these increase the cost of the termagants by 20%. Adrenal Glands might be worth it if you plan on charging with your termagants but since they are now not fleet you have to have them within rapid fire range the turn before to do it. Poison is good since your S3 unit now wounds every thing on a 4. Again strangley good for monstrous creature hunting. Are we getting the results of 2 hive fleets fighting it out right now. If you combine them both on the charge your Strength is 4 so you can reroll those failed to wounds on T4 or lower targets not to bad.
With both upgrades you can essentially go toe to toe with choppa Ork Boyz (Termagants generating .75 wounds each vs 1 wound each for boys) if you charge and they are not in cover since if they strike first they will clean your clock. These upgrades are not bad but the low weapon skill, toughness, and save mean the termagants will take heavy damage back. If they do not really hit the opponent hard and first you will lose the combat and then either be swept or no retreated to death.
Last Point to consider is the instinctive behavior: Lurk. So out of synapse you need to pass a leadership 6 test of go to Lurk mode. So you can pretty much assume you will be lurking a lot. Least you still count as scoring. Now Lurk requires you to shot at the nearest target if one is visible and in range or run to cover. No movement or assault. This is a place where the 18 inch range of the devourers/spikerifles might be useful since it might help you not run toward the nearest cover. I guess the thing to do is to make sure you put any objectives you plan on camping with Termagants in cover.