Ap Strings is very good. However, it has limited articulations, compared with the other Strings Collections, so there may be times when you can't get the articulation you require. However, it's worth the money just for the legato Cellos.
Do you also have the VSL solo strings? That might be the best library to go for next, as they blend perfectly with the Chamber Strings. Listen to that VSL chamber/solo string arrangement of the Bach organ piece on their demo site.