What are you guys using for string sounds?

I use the East West libraries but I doubt they do well in a fock mix. Never tried it. 

Garritan Personal Orchestra. I also use great old GigaSamples that I access with G-Player.

 
My violin and double bass, sorry couldn’t resist. :-)

dappa1
I don't think it matters its how you model the sounds and layer them. With the right reverb, you can get a good 3d shaped sounding strings.

I agree with dappa1.   The heart of Rock is the Guitar and Bass... the other strings mere trim.

Soundwise

Sijel
I agree with dappa1.   The heart of Rock is the Guitar and Bass... the other strings mere trim.

Not true!
 

Guitar, Drums, and bass. Or if you are a genius just drums and vocals.

If you have a little money to spend, consider Cinematic Strings. I think they're going going for 300.00. A great string orchestra, and super easy to use. Uses the free Kontakt Player. But, as has been pointed out, for Rock, strings are not as important and you certainly can get by with less.

If you know how to write music - and I mean you understand enough theory to write legato strings etc - then you can't go past Notion by Presonus. The sounds aren't the best in the business, but you can replace those sounds with a higher quality string library down the track - although I never have, because I believe if you know enough audio engineering techniques you can squeeze the sound into something more than passable. Even better you get full orchestral library. In fact, I write out all my pieces in Notion and then either import the midi in for drums, or import the audio files into Sonar if it's strings or brass. What I love about doing this way is you can really make the virtual instruments feel like a real person has played them, which is because you can write in dynamics and use the humanize function to push and pull the time. And Notion is only about 150 bucks Oz.   

There's also Kirk Hunter's Pop Rock Strings which has got good reviews. He usually has a good sale (about 1/2 price)once or twice a year. https://www.kirkhunterstudios.com/products/pop-rock-strings/

East West, and if you are looking for warmth instead of realism I use Omnisphere 2 and a Korg Karma.

I have been using a combination of several libraries ranging from Philharmonik 2 to some of the new strings in HALionSOnic 3. I love the Sonivox (Sonic Implants) Strings which I have used in their SoundFont format and they sound great.
Recently I have picked up Albion ONE and Output Analog Strings which I have not yet had a chance to install as I am trying to figure our my VST/Sample storage management.

My favorite so far is LASS, although they are not cheap or light on computer resources. They sometimes take some effort to program depending on what you want to do, but I've found them quite versatile and a solid choice for most needs. If you have money to invest and want to buy one library, that would be my top pick to check out. Listen to comparisons against Hollywood Scoring Strings, though, and see if you like that better. I haven't tried Hollywood Scoring Strings, which I've been told is very good.

I've also used Cinestrings Solo and East/West Symphonic Orchestra. Symphonic Orchestra is okay, but it's a pretty old library and just doesn't have much life to my ear compared to other libraries out there. Plus you don't get solo instruments out of it.
 

Session Strings Pro..

Vienna is good - if you get the right collection.  East-West probably makes more sense for this particular use.
I did not like ANY of the Garritan String Sounds.

The free included Cakewalk Studio Instruments Strings is all that you probably need for pop/rock backing tracks.  Next step up for cheap might be the Sonivox Orchestral Strings, available for $29.99 until April 2nd.
 
Garritan Personal Orchestra 5 is now 50% off at $74, until April 11th.
 
If money is no object, and you seek perfection or illumination, then the sky is the limit after that!