Just to wade in on the sniper front with a few examples from our campaign
I've been playing a sniper character in our DW campaign (technically 2 after an incident with a firewarrior squad finished my wolf-scout off) and I have used both the Needle rifle and the Stalker. My GM decided that as scouts are trained to use them all marines can use needle snipers, so my starting sniper weapon was the needle rifle.
Now having read the thread above i'll agree that its a fairly un-impressive statline for a sniper rifle HOWEVER when used to engage low-level enemy commanders (squad and platoon level (we are figthing Tau) or against smaller nids (after the Apothicary in the group played with the ammo) the needle rifle seemed sufficient for the task. Yes it lacks AP, but put a silencer on it and good luck to anything trying to find you. Put a scope on it and range becomes less of an issue. Take the time to aim and the extra damage dice are very useful. all in all a great weapon for dealing with soft targets.
After a few sessions my Wolf-Scout graduated to a Stalker, just in time for a full blown do-or-die extraction mission from a Nid held world.
I've got to say the upgrade was impressive. We have rulled that if a weapon gets tearing then all of its damage dice gets tearing, so my stalker tanked up on hellfire rounds aiming + a preysense sight was a rude shock for our GM when he sent a few big gribblies at us. First of he ambushed us with a lictor - which started trying to quietly drag our Black Templar Assault marine away. Being a wolf I noticed the lack of marine, spotted the lickter, and proceeded to 1-shot kill it with the stalker. Later on in the same mission the Apothicary managed to create some toxins and treat a clip of hellfire rounds for me making the end of mission battle a little odd.
We were holding out on the roof of an imperial facility with nids climbing the walls waiting for a thunderhawk to pick us and our objective up. About halfway through the holding action a Carnifex lumbers into view and splats a few npc's with a venom cannon. I very lucky roll later and mr fex is feeling less healthy thanks to the teams heavy bolter, allowing me to tip him over the edje with a single stalker round. Now the GM is clearly wanting to keep things interesting and so throws a Hive Tyrant at us a few rounds before the thunderhawk arrives. As everyone else is engaged in dealing with the hordes it fell to me to try and kill the Tyrant. Lady luck liked me that evening and 100+damage later Mr Tyrant has gone the way of Mr Fex.
Now move on a session and my wolf has gone to Russ's halls thanks to getting caught in the open by 25 firewarriors, so im now playing a Storm Warden who was training the 10th company scouts before his deathwatch rotation so is something of a recon specialist (something our group lacks)
Now im back to 0 renown again so its back to the Needle rifle, and in the first session I managed to take out a Tau Sergeant from 500m without the Tau being able to spot me.
In short I've used both the Astartes Sniper options, and I gm Dark Heresy so i've used my fair share of other snipers in the 40k universe, and if you use them as they were intended they are both viable sniping options - just don't expect a needle rifle to take out a light vehicle, for that you need a stalker and some kraken rounds - and don't expect someone to mistake the victim of a stalker for being asleep at his post or having died from an illness, thats what a needle weapon is for.