Originally Posted by
Anggul
Old Adversary is definitely worth it, and is taken by many players because of this. As Tynskel mentioned in his comments on Trygons, Tyranids are best when a few units hit at once. This means you'll quite often have a couple of broods within 6" of the Tyrant when it counts, and it also means the Hive Tyrant itself re-rolls most failed hits and wounds due to high WS and Strength.
In general response to what is being said here: Sadly, yes, there are only a few units worth taking. It's a bad codex, but we make the best we can with it. Almost all of the units are either really good or really bad. You do see varied lists, but it's true that most people default to a couple of Termagant broods, the Tervigons to go with them, Hive Guard and Flyrant. This is because in 40k you need Troops to capture, anti-flyer and anti-tank. These fit these roles perfectly and only have about one alternative each. For example I've seen armies based upon lots of big broods of Hormagaunts do well, and Dakkafexes do pretty well against flyers. Zoanthropes obviously perform a less reliable but more powerful anti-tank role than Hive Guard, and have the advantage of being massively flexible due to the rulebook disciplines. I use both, but many just use Hive Guard.
Basically you can make varied lists, but not massively varied without them just not working. So many things (Lictors, Pyrovores, Mawlocs, Tyrannofexes, Rippers and the now very sadly nerfed Genestealers and Venomthropes) are just bad and don't do what they're meant to either due to bad rules or points costs.