Prosonus - The Orchestral Selection comes in various formats. As a plugin you get almost 400 presets, a clever but uninspired interface from UVI and some cool sounding "real" orchestral sounds that sound (even in all formats) like the missing link between the mellotron and a library like Garritan Personal Orchestra or East West Silver Orchestra.
To clear up the contenders. GPO is the bargain. It comes with scoring software, a big library that is smartly designed for lots of articulations and actual orchestral playing feel. If you aren't a good keyboardist or don't need the much denser and more capable set you might want to skip it. You'll need to learn how to think like a orchestral instrumentalist when learning it. East West is a beautiful library but is lacking in things like smaller sized ensembles or solo instruments. Yes, there are solo instruments, but they are meant for use with a larger orchestra.
And here is where we come back to The Orchestral Collection. Honestly, I like it because it sounds so trashy yet real! The sounds seem to slide off whatever instrument you play; they have a greasy near wild kind of sheen. Don't even bother if you want to hear your latest symphony, unless you wanted to do it in lo-fi or punk classical...not that these wouldn't be cool things to do.
I see The Orchestral Collection as essential for film and television or rock bands that need cutting, slamming orchestral sounds. Don't overlook the generally overcompressed sound, it fits well with a lot of different music, just don't think of it as a serious orchestration tool.
Consider who markets it; BigFishAudio! These guys are known for some pretty wild stuff. I love their collections and have several. If you like big beat, nu-skool hip hop and techno, well you're getting the idea. The Orchestral Collection has all the subtilty of Webern on a particularly drunken day. But really, I love this collection for it's colorations to so many kinds of music and it's representational sound of orchestra while noises are going on all around.
Here's it's serious failing, and it's big imo. The UVI engine version has lots of fun stuff padding the presets but if you want to build your own sounds get the GigaSampler version. It's still lo-fi but with better clarity, even better for what you'll probably use this collection for.
I'm not sure what happened but it sounds like someone really screwed up on several patches in the UVI version. It's Limited beyond sense with sample flaws and what sounds like digital noise. Sometimes that's a cool effect but not when you can't get rid of it. In comparison the Giga version by way of Kontakt was much richer sounding and barely using any of the cool features you can apply using Kontakt or even Kompakt's truncated engine. There are still problems with samples scattered around this collection, but they aren't as offensive, and with a bit of your own creativity, you can clean things up so you could do some typical orchestrations. Listen to the violins in the upper range. If that grit makes you happy buy this kit. If not, consider moving on.
Pros: Exciting with a capital "E". While I love this collection I do find it almost offensive trying to market it as a all in one solution. It's interesting, it sounds great and blends well, or as they say, "sits in a mix!". This is the orchestra on drugs (or what people will say if you do a good job) and the effects in percussion, individual instruments and ensembles are effective. You can live your neo-classical fantasies, minimalism in post WWIII Canada where they eat people for protein, the would may go but carbs will stay avoided.
This is as much a synth as a orchestral collection and if you accept that you are going to have a great time with this set. I like it so much for it's style and near mellotronesque charm that I can't help but highly recommend it with the following caveats
Cons: Sample quality is more like 12 bit which is fine, but it's not for serious studies unless you can justify the purchase for classes and go and do your thing when you get home. While there are lots of effects and variations in sound and articulations the quality of the basic samples seem flawed. The UVI version is messy when working with 15 - 20 tracks. A regular sampler will keep things in order but some samples are missing. Many solo effects are gone and that's a drag. Compared to the UVI version the Giga is missing about 140 presets, most not available. It is a serious sin when libraries ahve different sound sets, albiet on the novelty patches, but it's a pet peeve of mine. When I get the hot shot format, and recommended by BigFishAudio, I expect that to be the big set, not the UVI library which is considerable lesser fidelity in a not so enjoyable way.
Still, I forgive BigFish, in fact, I love these folks. They make funky libraries that are fun to use.
Don't bother for "serious" classical. For any other direction worth a try and amusing for fun's sake.