I played against Tyranids for the first time in probably over a decade this afternoon. 1850 pts. I won the game thanks to seizing the initiative and then getting First Blood. I don't think their codex is bad, at all. I'm lucky to have won, it's one of the closest games I've had in a long time.
Guants with Devourers are half the points of a Marine, and each fire 3 shots? I'm not sure if that was something old or not, but holy crap. Throw a handful of the cheaper Spine Fist guys in the front and you're laughing off casualties, while laying down unsurpassed volume of fire from a Troops choice. I mean, what other troops cost so little and can fire so many shots from one model? Guardsmen with Lasguns, that get an Order from an Officer [similar to a "Blessing" psychic power] is the only thing I can think of.
On top of that, by shuffling broods of multi-wound models around, you can put a couple of wounds on this guy, shuffle the next forward so he starts taking wounds... oh, let's move the third guy forward to take the hits now. I can't think of any army that has that kind of resiliency [take damage, but still maintain damage output] built into so many models.
A pair of Carnifex that hit a vehicle are going to wipe it out. Like, "D" weapon type efficiency, even vs a Land Raider.
All of the psychic powers are useful, in a TAC kind of way.
All of the Warlord traits are useful, in a TAC kind of way.
I'm told that Warriors can be made to outflank. Shrikes are "jump" warriors that lose a bit on their armour save, but gain incredible mobility. Even jumping into cover is not that frightening, since they are multi-wound models, and wounds can be spread around your squad. A Mawloc that "misses" is still a big, scary MC that people need to focus upon right next to their unit. That increases the survival rate of incoming "slow" MC's.
Yes, Eldar can hurt your MC with their shurikens. But they have to get close to use them, and that's not so bad for Nids. They might get one, but they aren't going to get them all.
There are enough heavy hitters, blended with lots of horde wounds, mixed with disruption units, some fast bully units to pin enemy units, tied together with entirely useful psychic powers... Yes, they have a tough time when facing a lot of vehicles, but they always have. With missions forcing players out of their deployment zones, getting to grips with them has never been easier, even on a long board like Hammer and Anvil.
To the people that are down on the codex, I must encourage you to try it out. Hit all of the points above, and I think you'll have an effective army on your hands. I don't agree that the Nids have been given a raw deal, I think they just require a wide selection of units to succeed on the table top.