The purpose of this thread is to discuss the viability of tyranids in 6th edition. Have we gotten better? Worse? Or just different? I guess before we can answer that, first we need to know what has changed in the rules that have affected us.
First off, I will list some of the weaknesses that 6th edition has brought onto our nids. This is by no means an extensive list, just what I can think of at the moment.
Why We Can't Compete (Weaknesses)
1. Standard cover is only 5+ now. Of course you can still get 4+ cover from ruins, but the majority of cover - including cover from your own screening units - is only 5+.
2. Fleet - cannot assault after fleeting. What Fleet does is let you re-roll your run or assault move.
3. Cannot charge on the turn you come in from reserves. What this mainly hurts are outflanking genestealers as they can no longer assault on the turn they outflank.
4. Killing flyers. We'll have a tougher time taking out flyers because a) they cannot be assaulted and b) we are hitting them with our shooting on 6's (unless we get units with the skyfire special rule, which I don't believe we have). You can be sure the competitive meta will shift towards flyers.
5. FNP is only 5+ now.
6. Infiltrate - cannot charge on 1st turn.
7. Overwatch. Now opponents can opt to shoot at us when we charge, though they will be hitting on 6's.
8. Multi-charges. Now if you multi-charge, you don't get the +1A bonus nor any offensive buffing effects (i.e. furious charge).
9. (By Steve Hunter) No Allies.
10. (By Steve Hunter) Furious Charge no more +1 Init (only +1 Strength now).
11. We can't build an All-Reserves tyranid army anymore. Someone's got to start off on table and if you're not careful and he/they die, then you lose the game.
12. Challenges. Challenges can keep the enemy alive a little longer against your Tyranid Monstrous Creatures (TMC's) by limiting the amount of damage it can do to a squad.
13. Random charge lengths. This could be a boon to bugs as well, but for some reason, I can only recall the failed charges against units 3-4". Maybe it's just me, but I absolutely loathe the new random charge lengths, especially for an army that rely more extensively on it than other armies. Just 1 failed charge means that your unit will most likely get wiped out next turn to enemy shooting. Just 1 failed charge can mean the difference between winning and losing the game.
Why We Can Compete (Strengths)
1. Fearless. Now there are no more No Retreat saves. That means we have become a great tarpit army. They've also become a much tougher army to kill overall.
2. Flying monstrous creatures are awesome. Cannot be assaulted, moving 12"-24", only hit with shooting on 6's (unless they have the skyfire special rule) and can do a Vector Strike attack on anything it flies over. Flying MC's are the new hotness. Welcome back flyrant, we've missed you.
3. Cover. Though it's been reduced to mainly 5+, now it's also easier to get. You only need to be 25% obscured.
4. Assault is now 2D6", which makes our slower assault units somewhat faster.
5. Old Adversary. This has become a huge boon to us. Basically, Prefered Enemy now let's you re-roll 1's in both assault and shooting. Thus, I forsee the comeback of the 2+ walkrant with OA accompanied by hive guards and the t-fex nearby.
6. Now it is much easier to kill tanks (not including walkers or flyers). You either auto-hit them if they didn't move or you're hitting them on 3's if they did move, no matter how far they've moved.
7. Reserves. Now units will come in from reserves on a 3+ on Turns 2 & 3. On Turn 4, they auto-enter. Thus, reserve-based armies have become more viable. Note, however, that you can only reserve only up to half of your army (unless you're running units who have no choice but to come in from reserves like daemons or drop pods).
8. Though FNP is now only 5+, however, you can take it against power weapons and anything else with the exception of Instant Death wounds.
9. AP- now isn't -1 on the vehicle damage tables. Yeah, twin-linked brainleech devourers!
10. Though MC's no longer get +2D6 armour penetration, they can increase their strength to S10 (but they will only get 1/2 attacks, rounded up) and with re-rolls on the armour penetration. Coupled with the fact that vehicles are now easier to hit, I think MC's are better now at killing tanks. Oh, and also they get +1 on the vehicle damage charts because they are attacking at AP2.
11. Monstrous creatures now cause Fear and get an additional free hit (Hammer of Wrath) on the turn it charges.
12. Lash whips have gotten better. They are applied after furious charge, nemesis force halberds, banshee masks, etc. (from the tyranid FAQ)
13. Deny the Witch. Now in addition to Shadows in the Warp, nids have more psychic defense. Each unit can ignore a psychic power on 6+. Psykers (i.e. tyrants, broodlords, tervigons, zoans) ignore on 5+ and the Swarmlord will ignore most powers on a 4+.
14. There are some very cool new psychic powers. Now you can give the Swarmlord Eternal Warrior (Iron Arm, he also gets +D3 Strength and Toughness). There's also a lot of very nasty shooting attacks. You can also give your psykers what is essentially the Doom of Malan'tai's Spirit Leech ability (Psychic Shriek, but against 1 unit only).
15. (By Janitoria) Rage - now there is no downside to having rage. Instead, what you get is +2A. Go hormagants and carnifexes!
16. (By Genobraker2004) Boneswords. Unlike power weapons which got a big nerf against TEQ's (terminator-equivalents, or models with 2+ saves), boneswords will still kill 2+ units.
17. (By Genobraker2004) Glancing removes hull points which rocks adrenal gland gargoyles or gants wont have to eat explosions from rhinos anymore. And if you wreck it emergency disembark is 3 inches from a door maximum. (In short, furious charge gribblies can easily glance most transports to death without worrying about it exploding in their faces.)
18. (By Genobraker2004) Toxin Sacs on monstrous creatures. Now there is no downside to getting them for your TMC's. Poison now let's MC's wound based on their actual strength or 4+, whichever is greater, with re-rolls depending on enemy toughness.
19. (By Steve Hunter) Move Through Cover - most of our units, including genestealers, have it. That means we are not affected by dangerous terrain.
20. Now you can actually get cover for monstrous creatures in area terrain.
21. Concerned about Grey Knight nemesis force weapons? Iron Arm will not only potentially make your monstrous creatures T9, but it will give them Eternal Warrior as well.
22. Precision shots. That means the tyrant/carnifex with the dual tl-devourers has just become a deadly, sniping monster.
23. Barrages. Isn't it ironic that what is supposed to be one of the most inaccurate weapons in the game has just become one of the most accurate, sniping weapons? Yeah for sniping biovores.
24. 6th Edition Missions. 6E emphasizes objectives-based missions even more so than in 5th. Now of the 6 missions available, 5 of them involves objectives. Now there is only a 1 in 6 chance of getting our worst mission - Kill Points (now called Victory Points). Tyranids can dominate in objectives-based missions.
25. The Doom of Ma'lantai. With the nerf to mechanized builds in 6E, the meta is currently shifting towards more foot-based lists. Many players are starting to ditch their vehicles in favor of running more infantry-heavy lists. That means a much more target-rich environment for the Doom. Yeah!
26. Challenges. You want to keep your monstrous creatures alive a little longer in assault, especially against more assaulty (or deathstar) units? You can do so by issuing challenges and hope to get reinforcements.
27. Smash. I'm finding the TMC to be quite the character-killer with Smash attacks in combat. Whereas before, tyrants, tervigons and trygon primes could not easily kill enemy characters in combat, now they can with insta-killing Smash attacks.
28. Psychic defense. Most Marine armies have had their psychic defense nerfed because psychic hoods have changed in the new edition. Nids are 1 of only 3 armies (the others being eldar and space wolves) who have retained their psychic defense - Shadows in the Warp.
29. Better mobility. Flying monstrous creatures are huge and now gives tyranids the ability to alpha-strike their opponent, a better chance to get First Blood and to reach objectives in the late game in order to contest and maybe even to claim the objective.
There may be more strengths/weaknesses, but these are what I can think of at the moment. Feel free to add to this list. Overall, I believe the new tyranids have received a boost with 6th edition, but I haven't played them yet. Feel free to add your experiences with 6th ed. nids in this thread.