I've now played four times with my Tyranids at 1,500pts, and, following the sage advice of Gav & Darren, I've stuck with the same list in all four games. As they suggested, this has helped me to learn what each constituent part of the army does, and probably more importantly, how each unit supports and complements the others ('synergy', anyone...?).
Very briefly, my army list is:
HQ - Tervigon (with Catalyst etc)
EL - 2 x Hive Guard
EL - 2 x Hive Guard
TR - 10 x Termagants
TR - Tervigon (with Regeneration, Catalyst etc)
TR - 10 x Genestealers
TR - 11 x Genestealers
TR - 12 x Genestealers (with Scything Talons & Adrenal Glands)
FA - 3 x Spore Mines
HVY - Trygon Prime (with Regeneration)
To date, I've had one win and three losses, with all games coming against good, experienced players (my three losses were to Mick's 'Cheese-Fangs', the club-boss' Tau gun-line and a club veteran's CSM army). Firstly, and most importantly, the games have been fun! That has to be the prime consideration of any game. Secondly though, I'm concerned that I'm a little low on two things:
i) Tank-busting guns
ii) Early assault units
What I've found in all my games so far is that it usually takes 2-3 turns to get far enough forward to engage in any meaningful combat, and when the tattered remains of my units do finally get there, the opponent's infantry (most often Marines - it feels like I'm always playing Marines!) are still sitting tight in their Rhino's & Razorbacks. I only have four tank-killing guns in my 1,500pt list, so more Hive Guard or some Zoanthropes are an obvious addition, or possibly the big daddy Gun-zilla himself, the Tyrannofex. What are folks' opinions of this monstrous beast?
My other requirement is to tie up the opponent's big guns earlier, so I'm thinking about the addition of Raveners at the expense of one or two units of Genestealers. I would have to screen these relatively fragile beasts with a line of Termagants, but luckily the Big Mommas are more than up to the task of birthing screening units - indeed, my two Tervigons spawned over a hundred of the lil' critters! Have you, the readers, considered, or used, Raveners? What are your thoughts here?