The case
against Omnisphere, for those who don't have it and think maybe they should...
Nobody NEEDS Omnisphere. It's not a "bread 'n butter" instrument, neither as a synth nor as a sample player. It's not where you'd turn first for a bass or a piano. Strings, maybe, but although it has some very sweet-sounding strings you get no articulations like you would from a good Kontakt string library. The strings, like so many Omnisphere patches, are meant for beds, not as featured instruments.
Most Omnisphere patches are designed to sound good on their own, in solo. They'll typically have impractically-long release times and/or be drenched in reverb, delay and chorus effects. They are usually very broad-spectrum. Played in isolation, they make you go "oooh". But try to squeeze them into an actual mix, and they either muddy everything up or get lost. Most patches, unless featured up front or in solo, will require envelope modification, paring-back of effects, and aggressive equalization (usually a HPF) before they can be slotted into a mix.
Omnisphere's sample library is a closed, proprietary system, so there is little third-party support. The only aftermarket patch libraries available use only Omnisphere's synth engine, as they are not allowed to use its samples. So unlike almost any other synth or sampler, when you buy Omnisphere what you see is what you get. (Yeah, yeah, it's a big library! Not as big as my Kontakt collection, though.)
Omnisphere is a resource hog. You'd better have a top-spec machine if you want to run more than a couple instances. Sometimes, patches play OK but end up garbled after a fast bounce or export. Because many patches are so large, browsing patches during project playback may not be practical.
Finally, it's fairly pricey in the soft synth world. It's in the same ballpark with NI Komplete, which gives you a whole lot more content and variety. It's twice the price of Alchemy, arguably Omnisphere's closest competitor.
Hope those of you who don't have Omnisphere will feel a little better after reading this.
Now as for me...well, to paraphrase Charlton Heston, you can take away my Omnisphere from my cold, dead hands.