The comment about 10 tervigons in 2500 points - yeah. Disconcertingly tempted.
Had my first game last night, stick 1000 points of a friends Crimson Imperial Fists (20 tactical marines and 20 sternguard, commanded by not-pedro-kantor).
I used 4 tervigons and 3 12-strong units of termagants. Despite losing 50-ish, I had the better part of 70 left at the end of the game. Bloody hell they're awesome...
Anyway - 6th-specific comments:
Don't be scared of overwatch unless (a) foreboding, (b) lots of flamers (e.g. burnaboyz, who will **** you up!) or (c) massive unit (20 strong sisters of battle are now not nice to assault). As noted, disposable unit of termagants or rippers to soak overwatch are both useful and particularly nid-ish.
Losing the initiative bonus from furious charge (now strength only) hurts.
Challenges are awesome - either sarge doesn't get to fight, or he faces your tervigon alone, meaning you can stab him much more effectively.
Not seen vehicles yet, but suspect smash is a git for tervigons - although I imagine that WS1 means it's not as bad as I think - going from needing a 4 or a 6 to a 3 makes up for the 2 lost attacks, and S10 with a reroll turns walkers into scrap metal pretty damn fast.
New powers - not sure if you have to replace all the powers on a single model if you replace any. Biomancy, however, seems awesome. Firstly, they're all mastery 1, so they're all in play, and you have a 50% chance of a personal augmentation that (since it's not aimed at the enemy) can't be denied. Iron Arm is scary as hell on monstrous creature with S5 T6 before you roll!
Ended up tabling the marines, including an awesome three-round challenge against not-kantor*. Both sides bruised and battered (had a picture of termagants giving shoulder massages, fanning with towels and handing out energy drinks to both sides ) when Glorious intervention actually worked (rolled a 1!) and my warlord tervigon reached up and tagged a crushing claw to the Iron Arm tervigon, who elbow-dropped in off the ropes at S8 T9 to the sound of several termagants chanting "go kraken! go kraken!", which as you might imagine cleared things up rather fast.
Quite tempted at 1000 points to just go with 3 x tervigon & 3 x 10 termagants, plus a more expensive tervigon for the HQ - given the extra VPs for the warlord I'm thinking boosting for regeneration for him specifically isn't unwise... with six wounds there's a fair chance of getting to use it.
Stinger Salvos are as or more useful now than cluster spines, I'm thinking - the ability to snap fire with them and the ability to get precision shot probably makes up for the increased firepower's utility. Certainly it's a definite choice, rather than a near-automatic swap.
Warlord traits look cool, but the one I rolled was unfortunately fail - furious charge slightly pointless for the morph-laden tervigon. Can't decide which of the three tables looks best, actually...
* "...iiiin the red corner, weighing in at 250 kilos outside power armour, with several hundred career victories and one defeat, the hero of rynn's world, pedro kantor! In the blue corner, weighing in at four metric tonnes, just spawned earlier this afternoon, in its first professional fight, the smack 'em from kraken, the tervigon prime! I want a good clean fight, so no orbital fire and no spawning minions...."