I've seen a lot of theoryhammer on Tervigons with the forums saying not a chance, and blogs saying they're the best thing since sliced bread. I fall into the latter category, but perhaps not enough to run 5. I can see running at least 2 as troops, and perhaps one more as HQ. The reasons stack heavily in their favor.
3 words: Scoring toughness 6. Many people get hung up on the gant pooping ability, but I think that this is their real asset. The only other example of that in the game is full sized wraithguard, which isn't cost effective, considering that for their price, one can get 2 Tervigons. 12 T6 wounds is hard enough to shift, let alone the fact that said Tervigons will be pooping out gants. Which brings me to point 2
Gants, 3d6 of them a turn. Not bad. You should get 10 (+0.5), but even a tiny unit of 3-5 is perfect for going to ground on an objective. Do they shoot a whole dakkapred at 3 gants in cover? or the MC's, the other little bugs, or the flying bugs? The more choices you can force an opponent to make, the more likely it is that they'll fail one. Speaking of failing, many say that the chances of crapping out are too high to make up for their points. Not necessarily so. The odds of rolling doubles on 3d6 is 96/216 or 4/9. 44.4% chance per roll to crap out. Not bad, but certainly worse than I had initially thought. I think the key to tervigon is going to be knowing when to crap out and when to hold back. If you start to poop T1, you won't have 'reserve' gants in later turns. Hold back until you can spawn the gants into action. Being able to move and assault after a 6" 'deploy' can leave your opponets wrong-footed. When my league gets up in points, I'll have 2 Tervs, and roll just one poop per turn, hopefully getting some gants into the midfield to make a nuisance of themselves.
Powers. The fact that you can get a scoring psyker should rate these bugs higher on other's lists. FnP on a T6W6 beastie in cover means that it'll be there for turn after turn, and who doesn't like rolling FnP against krak missiles? The other powers are less noteworthy, but still have applictions. Dominion is alright if you need that squad just out of synapse to rally, or want your frontline to be fearless in assault, before you have time to move up. Onslaught is more use impaired. I guess your hive guard like it, and any bugs you want to keep moving and shooting, but out of the three, I like Catalyst most.
I suppose that for 200 points, people want a Face-smasher, but the tervigon is not such a unit. It has claws, and with the ag/ts upgrades you give it to share around, it can do alright against other troops, but elite CC units will make a mockery of your support unit. Once one gets it in their head that the Tervigon isn't a close combat unit, I think they'll fare better. You just need to work with it, not against it.
Of course, these are my initial thoughts. Once they get some time on the table, my opinion, especially about how many to run and when to spawn is liable to change.