My first two libraries for GigaStudio were the Presonus Orchestra and GOS.
When GS went away, I switched to Kontakt 2, specifically for the Strad and Gofriller solo instruments. (I was still limping along with GS on a dedicated machine.) GPO was one of my early Kontakt purchases. I did some good work with it. And it was a wonderful learning experience.
As GPO (and JABB) went from one delayed release to the next I lost faith in Garritan the company.
I still don't make a ton of money from music production, but I have purchased a handful of the big(ger) dollar libraries, and I can tell you that they have trimmed hours of my time to complete a project. Don't kid yourself, they aren't perfect - at least none that I own - but they do let me work much more efficiently, and to my ears they do sound better with minimal effort.
So I would suggest, to those that think the higher priced libraries are in fact over-priced - if you are charging for your work you might want to take a look at them.
Specifics, for those that are curious...
I use Cinematic Strings 2 and 8Dio Adagietto and Agitato for most string parts. The dynamic bowing patches in Adagietto can be absolutely magical - I do with they would tempo sync though. And for more contemporary stuff I still find Kirk Hunter's Pop/Rock and Concert Strings libraries very useful.
I use Cinesamples core libraries for classical brass and all my woodwinds. I also still use Sampletekk Big Orchestral Brass to supplement.
I use Chris Hein's Compact Brass for pop stuff. I've also been toying around with Big Band Brass (a loop library), but thus far I haven't been all that thrilled. That might have been a poor purchase decision.
I think I have about 1 gazillion percussion libraries - I've lost count! But I use various percussion bits in just about everything I do. The Tonehammer/8Dio/Sound Iron libraries do most of the heavy lifting, for contemporary hand percussion I'm using Brio and Flying Hands a lot too.
I also have a bunch of pianos, but I find myself returning to the Sampletekk pianos most of the time. The Rosewood Grand from Orange Tree also gets a lot of use.
About two years ago I picked up a copy of Orchestral Essentials second hand, I've long been curious about the entire ensemble approach. For the better part of a year it just sat there and mocked my attempts to coax music from its bits. And then I started to get the hang of it. And once I did I became even more curious, so I picked up Albion 1.
I still turn to the more conventional libraries most of the time, but if time is a problem I can generate a pretty passable track using the ensemble approach. And I use them a LOT to add some sense of space or something (that magical glue).
And then this past fall a friend made me an offer I could not refuse - I was looking at Vienna Ensemble Pro and he suggested I try the basic Vienna libraries as well. It was a thinly veiled ploy to sell me his copy of their Special Edition Vol 1 (he had recently upgraded to their all-encompassing bundle).
This is not a simple instrument, or workflow, but dang, it is powerful! And the instruments sound great once you treat them with a little space (early reflections and reverb). Heck, they can sound gorgeous, and they layer really well with my 'older' libraries.
There are, of course, a number of other libraries I'd love to try, but these represent a material investment, so for now I think I'll continue to work with what I have.
Anyway, my point in all of this is that you can do great things with GPO - but I think you can do even better work, and work more efficiently in the bargain, with the more complex/expensive libraries. And they aren't THAT much more expensive.