I have recently become a proud owner of GPO 4 and am delighted with its capabilities. Pity I did not have it earlier. I spent quite some time trying to curb Kontakt 4; its library does have some good instruments but I was often exasperated being unable to render just the sound I wanted. It might have been my fault after all but the fact is, the same me finds GPO much more manageable and predictable.
GPO has helped me finish the following piece that I started while still on Kontakt, from which I have retained French Horn and Bassoon - unfortunately I could not quite find the desired timbre for the former in GPO.