Played another game with Marines vs Nids and the game went a lot better with both of us feeling the Nid list is not too borken, but does need slight tweaking (OP in places, UP in others - balance? maybe) - we are, however, now 'on board' with the Tyranid list

Marines:
T.Hawk - 2 x Devestators w/Librarian
Cruiser - 2 x Tacticals w/Supreme
Terms w/Chaplain
Landspeeders
Scouts w/sniper
Warhound
Devastators w/1 razorback
Tyranids (iirc):
Haridan, 8 Gargoyles
Node, 4 Dactylis
2 Warriors, 1 Tyrant, 1 Trygon, 4 Horma, 8 Term, 3 Haruspex
2 Tyrant, 1 Term, 1 Horma, 3 Malefactor, 3 Carnifex
4 Warriors, 3 Malefactor, 8 Term, 4 Gargoyle, 4 Horma
3 Lictors
6 Genestealers
Not optimal lists, for sure, but a few different formations were being tried out to see.
A total of 10 assaults were fought, with both armies winning 5 each. However, when the Tyranids won they won BIG meaning that the Marines were wiped out more often than not. The Marines, for their part, could not win the assaults by big enough to make any formations 'combat ineffective' and could not get to the Synapse (those that did die in the battle were caused by Landspeeders). The Tyranids, considering they were meant to be weak in FF, would often put down a horrendous amount of dice (all those Termagaunts!) - one FF was against my 'on the ground' Devs with Warhound and the Tyranids won convincingly (wiping out the Devs) as they had 16 attacks in the FF!
Harridan:
Problem - this beast is really over-powered in out opinion (in our game it charged and wiped out a Tactical formation to a man and then sniped at other formations with its massive range). It has done horrendous amounts of damage in every game so far - its fast speed and 6 attacks (2 MW) make it horrible in assaults. Our Tyranid player is apologetic in using it as it is so powerful (not counting the Gargoyles it carries) and now it is not Synapse, killing it does not effect the brood with it.
Solution - get rid of MW attacks, (and reduce firing range to 30cm). It should be a harrasment brood, really, and it is currently powerful enough to make head-on attacks.
Blast Markers:
Problem - Tyranids have far too many 'tricks' in this department. The problem is presented thus,
1. brood creatures do not cause blastmarkers when they are killed
2. broods do count for number of units in formation vs breaking
3. +2 to rally test
combined, these are far too powerful, in our opinion, meaning that shooting, which should be the achillies heel of Tyranids, is comparativly ineffectual. Tyranids should be able to keep coming despite enemy fire, but the current system makes them too powerful in this regard.
Solution - not too sure, to be honest. Tyranids should be 'slowed' by concerntrated fire, but it is difficult to achieve this without 'gimping' the Tyranids in other ways...
Initiative:
Problem - with all the special rules to do with Tyranids, they often achieve their tests on a 0+ - they can rally better than Space Marines!
Solution - lets call a spade a spade and give them an initiative of 1+ and get rid of all the initiative rules. The Hive Mind will control a hormagaunt to assault as much as an Exocrine to shoot, so why different rules?
Dactylis:
Problem - a very weak unit. While Tyranids should not excel at long range shots, this unit is not really worth taking it is so weak.
Solution - the initiative increase to 1+ will help this unit somewhat when on its own (with a node, for example), it should also increase its FF to 5+ to make it more worthwhile to include in amongst other tyranid formations. Or reduce its points to 100 for 2.