EastWest/Quantum Leap Symphonic Choirs is an excellent solution. Mainly because of the WordBuilder it comes with, but it delivers an exceptional quality nonetheless, as EastWest/Quantum Leap products do most of the time.
With older versions, WordBuilder is sort of an extension that lets you write lyrics for your virtual choir. In the newest version they integrated it into the main interface. It can be a lot of work fine-tuning vowels and consonants so that the result sounds natural, and it probably requires some talent with phonetics, but once you get used to the special markup language you need to input your words in, it can get pretty realistic. You might want to check out some video tutorials on how it works and see if you can get along with it. Even though WordBuilder is Symphonic Choirs' main strength, to use it is only an option, or at least in was in the older versions. I didn't use the newest interface, there might have been conceptual changes, but as far as I know, when you're not interested in lyrics, you still have full control over everything else. Older versions use the supposedly familiar Native Instruments interface, the new one uses PLAY which is quite similar.
It's pretty important to note that, as for the sound, EastWest/Quantum Leap orchestral pruducts always seem to pursue a Hollywood approach if you know what I mean. They use miking techniques to deliver more of a fashionable, commercial sound rather than a classical naturalness. It comes out mostly when comparing EastWest Symphonic Orchestra to Garritan Personal Orchestra, with the latter sounding raw and real, while the former sounds like a movie with all the close miking, sometimes a bit too crystal clear. Unfortunately Garritan doesn't sell a standalone choir as yet, only a "Full Choir Lite" as part of GPO4.
Having said these, as with everything, it's always best to give it a free try and listen to what it can do.
