I really like the model with its hunched, menacing thorax, bullet shaped head and club like tail. I didn't want to change much apart from the weapons and posing. I converted mine into Dakkafexes by re-purposing the Deathspitter arms in the box with Termagant devourers. I cheated a bit by leaving a pair of scything talons on the model but this is only for show. The devourers make the beast more expensive but they are still much, much cheaper than the old 5th Ed versions. Its not exactly an original idea but I found that swapping out scything talons for 2 pairs of twin linked brainleech devourers to be more versatile. Now effective at range, the Carnifex can still peel open a tank like a banana. I also prefer the Living Battering Ram rules in 6th Ed. and the slightly increased initiative. They lose an attack without 2 pairs of scything talons but 3 attacks is the same as 4 when you are charging and smashing. I magnetised all the devourer arms, just in case. Better to be safe than sorry.
I've modelled mine with Thresher Scythe and Bone Mace tail biomorphs but I'm not actually using them. I think tail biomorphs are a cool idea but they aren't particularly good in practice (especially when you forget to use them) and I want to keep my MCs as cheap as possible. This is because they are going to die. Its in the nature of Tyranids to get blown apart in messy gobs of alien ichor.
Ironically, spamming cheap MCs makes them more survivable. Sure, my opponent shoots at them in turn 1, but a strategically placed Venomthrope helps to mitigate that. By turn 2 he has 4 flying monsters barreling down at his troops (Comms Relays are awesome). Killing my comparatively squishy Warlord flyrant or grounding and pounding those pesky, vector striking Crones is often a more pressing need. And I've never met an opponent yet who could resist atomising huge swathes of Termagants with template weapons. The Carnifex is often ignored in the crush of target saturation. And its easier to hide out of LoS than other MCs.
I run them both separately rather than in the same brood because I don't want them to eat each other. The instinctive behaviour: 'Feed' is more forgiving to lone monstrous creatures. I probably don't need to be that cautious since it amounts to play fighting where Carnifexes are concerned. They have a fairly low chance of actually damaging each other with such thick hides. In truth, they haven't had to make an instinctive behaviour check yet because of my extremely cheesy interpretation of synapse rules for creatures inside buildings and lavish use of the 'Dominion' psychic power (I'm loving my HiveStorm Redoubt so far, even when it was blown to bits by some cheeky demo-charge guardsmen in my last game, killing the Zoanthrope inside and scaring the piss out of the Warriors on the battlements)