abacab
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.

I was just about to suggest both of those. Studio Instruments Strings are surprisingly good. Sometimes you can catch the Sonivox strings on sale $19.
 

I use various Vienna Symphonic library collections. Appasionatta, Solo strings, Dimension and the New Synchron strings. In combination with Vienna Instruments Pro and MIR Pro it's hard to go wrong. They are well suited to provide realistic Strings that cover any style of music. Classical, Pop, Rock, Hip hop, Funk. Eay to reprogram the matrices to suit your playing style or articulation needs. Humanizing Tuning and Delay scaling is very magical and organic. Articulation selection via Ipad app is very nice too!

The SI Strings is amazingly versatile - especially for a bundled instrument. It covers the pad spectrum at low-medium velocities, and the bows really seem to "dig" at the higher velocities.That can actually be a problem sometimes, for example if you just want a basic fill that doesn't stand out, but your keyboard controller is touchy. SI can feel like it's wobbling between sticking out and dropping out. For some tracks I've used MIDI FX as a sort of "MIDI Limiter" to narrow the key velocity to just the range that the part really needs - works like a champ.
 
But as others have said, SI really shines when it's layered with other parts.

losguy
 
But as others have said, SI really shines when it's layered with other parts.

It probably really shines when you save $1000-2000 trying to add some backing strings to a rock track! 

 No joke, abacab.
 
Though, I think I caught a whiff of "stinkin' rock track" in that remark, but I nonetheless acquiesce and will strike that from the record, your honor.
 
Like a handyman friend of mine once said, "You want the right tool, for the right job, at the right time."

losguy
 No joke, abacab.
 
Though, I think I caught a whiff of "stinkin' rock track" in that remark, but I nonetheless acquiesce and will strike that from the record, your honor.
 
Like a handyman friend of mine once said, "You want the right tool, for the right job, at the right time."

Haha!  Not me!  I love rock, and most other forms of music!  But I do have friends who believe the only rock instruments are guitar, bass, and drums.
 
I lean towards the more progressive forms of rock, in styles like Genesis, Pink Floyd, Yes, etc.
 
Although I think that the Phil Spector 'wall of sound' https://en.wikipedia.org/wiki/Wall_of_Sound can get a bit overdone at times with pop/rock... 

Except for that first Genesis album (From Genesis To Revelation) where the strings were WAY overdone...