I admit it, I'm trying to like the Carnifex in the codex. But, the question is, what do I use it for? With the plethora of new critters/options, it's now a good question as to just what you CAN do with the 160-point bugger.
What Does it Have Going For it?
Well, you have good ol' Monstrous Creature durability. T6, four wounds, and a 3+ armor save. And, now, you can take 1-3 per slot (more on that later...). You start with four WS4, S9, I1 attacks (I3 on the charge, and you're the one thing in the fething book that can take frags; go figure). You start with double scything talons, so you can re-roll anything you don't tag, and you'll generally hit on 4's (barring vehicles, crappy troops, or assault troops). Lone carnifexes can also take mycetic spores.
What Can it do?
Now, here comes the real question: just what are you going to DO with the carnifex? Basically, you're going to think about either shooting OR assaulting, since you can't do both at the same time. Furthermore, buying guns loses you one or both sets of scything talons, which seriously impacts your ability to wreck things in melee.
Note also that in a unit, each 'fex must be equipped identically; I guess there's just too much room to abuse three seperate carnifexes for wound allocation. (Then again, they could theoretically soak 10 wounds before losing ANY combat effectiveness, so yeah.)
So, for shooty, the options are:
1) Take a Heavy Venom Cannon.
It's reasonable for suppressing fire, as S9 can usually get hits on vehicles and slow them down. On the negative side, it's still -1 to damage rolls and you're only BS3.
While you're CHEAPER than a Tyrannofex (he's 265 with his rupture cannon, you're pushing 200) he's much better at killing armor with two S10 shots, and landing at least one of them 75% of the time.
2) Take a Stranglethorn Cannon
Mmm, S6 large blast. Ok, doesn't do crap-all against vehicles (well, nothing that a Hive Guard can't do better and cheaper) and it's okay for anti-personnel work, but that's still an 'eh' weapon, all things considered.
Bright side? You're twin-linked. Down side? You're short ranged (almight 18") and not going to hurt vehicles at all. If you're committed to going shooty, take them to get a little extra oomph (points permitting) and if not, well, honestly, you should have more efficient ways of killing infantry.
When it comes to assault:
1) Crushing Claws
...leave them at home. 25 points for +d3 attacks. You have four with re-rolls, or you can get 5-7 without re-roll. You need dice to go in your favor to get any mileage out of this; with 4+ to hit you need to get 6 attacks to get mileage; with a 3+ you need to get all +3 attacks to make it work for you.
2) Adrenal Glands
Yay, Furious Charge! IF you actually get the charge, you got at I4, S10 (since you get +2I on the charge for being a really big, angry thing). The only time you'll care about being S10 is against Wraithlords and AV14. AV10 rears get auto-penetrated by S9, and you should be able to tag AV14 on average; 2d6 usually gets a 6-7. Leave 'em at home.
3) Frag Spines
...if you have five points, I guess? Mostly, this is for getting to go before stuff like units full power powerfists, since you still don't go before dreads (unless you bought Furious Charge). It's cheap, so it falls into the filler category.
4) Toxin Sacks
You're S9. Unless it's a wraithlord, you wound it on a 2+. This beats a re-rolled 4+. You already nuke multi-wound T4 on a 2+. You don't need these, frankly.
Now, Why Would We Use This?
So, you want to shoot? The Hive Tyrant is comparable in terms of weapons and BS, and brings you better support abilities for the guns you can get. In a pinch, you can ALSO get tyrant guard and get more durability.
If you want to kill heavy armor, the Tyrannofex is more expensive, but has a better gun for killing heavy tanks. It also has nastier point-blank weaponry, a better save, more wounds, and loses one attack and one WS (and is only S6, but BIG DEAL).
If you want to drop-pod it, you're inferior to the trygon. It's faster (thanks to fleet), it's got a gun (for what it matters; BS3 with assault 6, S5, AP5), more attacks (6) and it has more wounds. The spore gets a gun, but gives up a KP (or VP) as well. Frankly, I'd consider going with the Trygon here.
So, then, what would we use the carnifex for? Stuff shoots better and deep-strikes better. Also, Warriors are a source of power weapon attacks (though Warriors are more expensive) and a Tyrant + guard CAN be a nasty counterassault element. S9 insta-kills most multi-wound stuff you'd want to insta-kill. So, it IS a potential substitute for Warriors and nasty counter-assault.
My thought is thus: Keep them on foot, MAYBE take frag spine if you have them. There's no benefit to the assault upgrade, and there are better guns.
Run them at the enemy as a target. Bring along either hormagaunts or termagants as a means of KO'ing infantry, bring some guns to take care of enemy tanks, and go to town. I don't think it's necessarily the best way to go, but I think you might be able to work it.
I would start with something like this:
6 Hive Guard (for killing light transports; could go up to more) 
2 Tyrannofexes (for long range fire) 
Unit of 2 Carnifexes (no upgrades) [320ish]
This is about 1,000 points. You've got a good set of four monstrous creature; I'd think about more. You've got the means to dismount the enemy infantry; you just need synapse.
An Alpha hiding in the hive guard is an option. Alternatively, a couple of Tyrants could be nasty mobile synapse; if you're going with hormagaunts you could try to give 'em Preferred Enemy.
Tervigons boost your MC count, at least.
Anyone else got any thoughts on using Carnifexes? This is the best I've got. When I get back to my codex, I'll crank out a list. Again, I'm not sure this is the best way to run bug, but it's what I've got. Throw the Carnies out there to draw fire; if they CAN get into assault they can do some damage.