The Trygon weighs in at a smidgen over 200 pts (with adrenals) and is one of the biggest models I have seen in 40K, but you get a model that has more 6's in it's stat line than any other; is fleet; can deep strike; has a gun (6 shot lightning bolts from it's face!); and rerolls all misses in cc. There isn't much this beast can't do, thus we shall call him Mr. fix-it. I would also accept "Big Kahuna", "Mac Daddy", or "that damn big thing".
The Tervigon while being big and brutish on the outside plays more of a support (almost motherly) role. With its ability to give a unit feel no pain or allow a unit to run and shoot; birth 3d6 termagaunts each turn while also giving them furious charge, 4+ poison, use of the Tervigon's Ld and counter attack (and did I mention it is synapse), this little lady takes family fun to a whole new level. As a bonus, each counts as a troop choice for every unit of termagaunts you field.
For those of you wanting to convert a Carnifex to a Tervigon, I added another back plate with the spines (adrenal bug underneath it), a row of center spine plates, the little talons from the Mawloc, rending claws from warriors (now just arms to help with birthing), and expanding foam on the underside to be the egg sac.
As far as tactics goes, I have found that starting with both Trygons on the board and heading straight into the fray works best. Due to their size and combat potential, they draw fire away from other key units. Against IG or Tau, I might consider DS one of them. The Tervigon's tactics are a little more tricky since you have to juggle flocks of termagaunts as you stomp across the field. The buff's only work out to 6" so you have to pre-plan your unit position or you may end up with gaunts outside of 6" or a Tervigon outside of charge range, etc. I also take a Hive Tyrant with the Commander upgrade which adds 1 to reserve rolls and allows for one TR choice to outflank. Since the Tervigon is a troop choice, I now have the option of sending one of these lovely ladies in from the side - a nasty surprise in table quarter deployments.