My Tervigon conversion is extremely close to the real thing.
Here's mine:
And here's the GW one:
Pretty close if I do say so myself! Notice the Termagant coming out of the birthing sac, the spines and the fact that it's walking on its claws. I also almost used the Carnifex head with the tongue. Granted, I used the codex artwork as my inspiration, but I think I did pretty well for myself.
The differences are the height of the legs and the sheer size of the birthing sacs. On mine, I enlarged the belly but this one is just Octomom massive. Mine will also have an easier time getting a cover save.
Here's the link to the original post where you can see different angles of my Tervigon.
/shameless boasting