The Hunting Rifle is both the most over and under-rated weapon in the game.
For the uninitiated (the initiated are free to skip this paragraph), L4D is a first-person zombie-apocalypse shooter. You and your three friends roam the city trying to get from safe-room to safe-room until you can find an evacuation point. You have a primary weapon and a pistol (or two) with unlimited ammunition as a backup. Along the way you'll find three "good" weapons, almost always grouped together: an automatic shotgun, an assault rifle, and The Hunting Rifle, which has a smaller clip and slower fire rate, but packs a mighty wallop and can be fired accurately over a great distance.
Hunting Rifle lovers point out the strengths: you can take down a Tank or a Witch at a distance without having to reload (on Easy or Normal, anyway) and that bullets pierce multiple enemies (the shotguns do that too, but have a far shorter range of effectiveness). What I found it most effective for was clearing out big open spaces from a distance. We've all done it--you come around a bend and find a large open area with maybe 30 infected in it. With a Hunting Rifle you can clear that out in about twenty seconds. Without one, you get close enough to start picking them off and the rest mob you.
HR detractors will point out that the weapon is basically useless in close quarters or against Common Infected on the move. And they're right. Even with the extra powerful bullets, it is far less useful than an automatic shotgun, which also pierces and has a wider target area. When I played with the HR, if the action got too close, I found myself switching to dual pistols, because I could fire twice as many bullets in the same span of time (doubling my chance of hitting anything--you can't take careful aim the middle of a mob) and reload just as fast (if not faster). But perhaps the biggest issue with the HR is that it lends itself to distraction--it's very tempting to use the scope to pick-off far off targets, but there are often better ways of dealing with them (pipe-bombs or simple avoidance come to mind). And waiting around while someone plays "sniper" can cause the AI Director to send another horde at you.
So you have the odd duality of the Hunting Rifle: it is a sublime asset in some situations, a liability and a distraction in others. So I think it can be used effectively, but the key to its use is in knowing when to not use it. Don't charge to the front and start sniping--hang in the back and snipe when needed; use your pistols the rest of the time. You also need to have a team accustomed to fighting with a sniper who will call him/her to the front when the time is right and protect him/her when the horde arrives.