Funny GPO would come up... actually not funny, but I just reinstalled Aria, GPO and JABB again last night.
I'm currently using libraries from KHS and Project Sam, and the quality (not as in good or bad, but as in tonal quality) is quite different! I'd have a difficult time going back, and in fact plan to go forward with LASS or Cinematic Strings or Spitfire or Cinesamples or... well you get the idea.
HOWEVER... I do not think there is a better tool than GPO for learning electronic orchestration! Part of that is that you do not have dozens of articulations, so you can focus on choosing the right instrument and articulation without pulling your hair out. A bigger part is that you can, and really must, build you ensembles from individual instruments.
They are never going to compare favorably with libraries costing 10 times as much - nor should they. But check out the magic that some folks have created with nothing but GPO, some mad orchestration skills, and some killer mix skills.
I still recommend it as a first library when I'm thinking straight!