bitflipper
 
To get back on topic, I agree with Mr. Baer above. Omnisphere is far and away my favorite synthesizer, but if I had to choose just one thing to take to a hypothetical desert island (that somehow had electricity), it wouldn't be Omnisphere. It would be Kontakt.

Which is why my recommendation to the OP was to check out the free Kontakt Player, as well as the Sampletank CE, to see what he thinks about them.  Kontakt would be the best option if you plan to collect sample libraries.

So much depends on what you want to do and advice depends on where the advice giver is in their musical journey and the biases they have developed.
 
For orchestral stuff this is the best short guide ever made...
https://stormsoundmusic.com/guide
 
Everyone who is in any form serious ends up with Kontakt. Try to do it at a substantial discount and consider Komplete.

Bigcat... Zao's guide is very incomplete...and dated... yes, good stuff but wouldn't use it as a buying guide as includes many dated and little of the recent things like inspire  or TO... is newer better? In this case, YES! GUI design, sampling techniques, scripting...all have improved to help make libraries more playable and realistic, less CPU/ram memory/SSD intensive...by average players like me
 
Re Omnisphere vs Kontakt? The free Kontakt  player gets you the orchestrals and O2 gets u the world. Is Kontakt in nearly every orchestrators/songwriters arsenal? Yes, as is O2... Kontakt is a far better sampler but there it ends. My point is you get the high end content that would cost thousands to try and replicate with K (a pretty empty vessel on its own...and yea, komplete/ultimate gets u loads and it's the "wow! Bargain basement effect that's gotten so many of us to buy in but it's all over the place, gui/quality/feature wise AND learning curve is high for all the zillion so so things which aren't  in omnisphere's league.
 
I know there are lots of komplete fans here...heck, I have Kultimate  myself... but the best thing in it,by far, is kontakt however, even here, each library is different, gui wise... not to mention quality and cost...its an endless moneypit for quality stuff
 
Just pointing a few things out to someone who might wanna focus on composing vs endless, "Wtf is this and how do I use it?"s and disappointments...in this regard, O2 excels, as does it's stellar 60 gb library and unparalled effects plus ability to layer/modulate everything with a consistant engine like no other.
 
Omnisphere+inspire+The Orchestra... top notch, state of the art weapons with NO garbage, confusion, wasted $$$/gas regrets...for $1000!  The rest can come later...and itll all be different by then, but that is an immediate arsenal, and when married with all the free Skippy education vids and additional content... heaven...
 
Just offering a contrary pathway not mired in "history" that avoids the endless gas expense, offers the most bang for the buck, modern/efficient/useable GUI's, top notch quality and immediate usefullness to a musician/songwriter.
 
https://www.pluginguru.com/products/
https://www.youtube.com/user/thepluginguru
 
I'll go away now...

OP again - thanks all for your fascinating input so far...
 
I watched some Groove3 videos last night on both Kontakt and Omnisphere. After watching the first three on Kontakt I was beginning to lose the will to live - it just sounded so complicated. But then a few pennies began to drop, and I did start to get it - a bit. 
 
I do have the free Kontakt player by the way, and am continuing to experiment with it before making a decision. 
 
I then watched some Omnisphere videos and somehow it looked so much more logical!
 
There is a massive difference in the GUIs - Kontakt to my eyes looks so dated and clunky, whereas Omnisphere is bright, well organised, easy to follow and modern.
 
Of course it is not primarily about the GUIs, but the sounds and the functionality at the end of the day. 
 
Back on Groove3, Eli Krantzberg (for whom I have massive respect) sounded positively downhearted  as he went through the technical stuff on Kontakt - just my impression - it's just so utilitarian looking. He also does the Omnisphere series too, and was sounding positively chipper by comparison! 
 
I know I said I was looking primarily for real world instruments but great sounding synths would also be a really useful addition to my music making. 
 

bitflipper

abacab
But be honest,  that it is the company you have a grudge against, and not the product.  ST3 delivers what it claims to just fine!

I have no grudge against IKM. It is the product, not the company.

Same here, while there are some nice sounds in ST3 I can almost always do better elsewhere. It was a disappointment and I usually look to Kontakt first, then Composer Cloud, UVI and Rapture Pro

Amplitube on the other hand is the biz.

Just to round this off, I have decided to buy Omnisphere first, and when Kontakt goes on sale (perhaps Black Friday) I'll maybe pick that up too.

I've been blown away by some of the sound demos I've heard of Omnisphere, including a couple by one of my heroes, Jordan Rudess at NAMM.

I know this is somewhat at odds with my emphasis on real instruments in my original post, but I think if I pick up Kontakt as well and add to it if I need to, I'll have most bases covered. I'm not trying to compose a classical score.

I really appreciate all the time folks have gone to in enlightening me in this thread. I've certainly learned tons.

Jordan Rudess is the one who sold me Omnisphere, too. Beware; once you get sucked into the Spectrasonics world, Trillian and Keyscape will also start calling out to you.

Check out Jordan's GeoShred app. Remarkable.

Marshall
Just to round this off, I have decided to buy Omnisphere first, and when Kontakt goes on sale (perhaps Black Friday) I'll maybe pick that up too.

I've been blown away by some of the sound demos I've heard of Omnisphere, including a couple by one of my heroes, Jordan Rudess at NAMM.

I know this is somewhat at odds with my emphasis on real instruments in my original post, but I think if I pick up Kontakt as well and add to it if I need to, I'll have most bases covered. I'm not trying to compose a classical score.

I really appreciate all the time folks have gone to in enlightening me in this thread. I've certainly learned tons.

Let us know how you get on with Omnisphere! 
 
I have held off, mostly due to the cost, and that there is no demo version.  But the demo videos do look cool, and a lot of folks seem to like it!

I took delivery of Omnisphere today. Obviously just been flicking through some presets and stuff so far, but one expression comes to mind - JAW DROPPING. The last time I felt compelled to use that expression in relation to music software was when I first got Melodyne 10+ years ago.