Yes, I'm giving this a 10. Not because I'm a fanboy, but because it honestly deserves it. Read the bloody review before you say "not helpful" just because I gave it a 10.
Omnisphere is, in my opinion, one of the five best synthesizers ever created. Ever. Hard or soft. It's that good. I have owned it for about five years now, if memory serves, and I use it on pretty much every track I write (and, for the record, I tend to write Berlin School and IDM).
...Now, having said that, do you need it? It depends on what kind of music you write, really. It tends to have a very particular sound that, honestly, can get a little redundant. More accurately, it has a few sounds, and it definitely gravitates toward those sounds.
Hmmmn, this is actually a little hard to express. Obviously, Omnisphere is a very flexible synth! The collection of sounds Is decidedly diverse, and of extremely high quality. Of course, some sounds are "better" than others and some are certainly one-trick ponies... and the presets make use of those one tricks well enough, so you'll never use them yourself. And the modulation matrix is probably the best mod matrix you'll ever use: everything connects to everything (including, VERY usefully, the effects), and you get an incredible number of slots to work with (I have never run out), AND it's usefully visual, allowing you to alter param values right from the matrix: great touch
There are a few gravity wells of sound that you'll find yourself falling into with Omnisphere. ...Or, more accurately, that I find myself falling into: YMMV. One is a Virus-like arpy sound, created by modulating the start-time of a ROM setting. Omnisphere has a lot of superlative synth sounds, and they lend themselves to arps. ...And of course the built-in arpeggiator is (again) the best you'll ever have used. Omnisphere handles trance and Berlin School with finesse and style: if you write this type of music, you'll want to buy it, period. However, when you start applying delays and autofilters and built-in reverb to them, they will start to "sound like Omnisphere". The ability to modulate the attack to add noisy perks also ends up steering you toward a very Omnisphere-sounding noisy attack. There's something about it that is unique. It's great! Don't get me wrong... but it's identifiable and you may get sick of it (a little) eventually.
A word on these sounds, though: I own and use ElectraX, Largo, and Diva: they sound better at the sounds they make. Omnisphere has some great old synths, but Diva sounds fuller and richer because of it's exceptional filter. Omnisphere has GREAT digital bells and attack sounds, but ElextraX is edgier and more versatile with them. If Omnisphere has a comb filter (off the top of my head, I don't think it does), it can't compare to Largo's ability to sound metalic or plastic (which, sometimes, you want). I find that Omnisphere supersedes the other synths when you want something--bear with me, here--either milder or harsher than the other synths. For example, if I want a gentle arp sitting in the background, I'll reach for Omnisphere and dial up, say, a Farfisa sound and turn it into a pluck (which I highly recommend, BTW). Sounds great, doesn't hit the CPU too hard, easy to modulate with the Orb for variation over time. ...Or if I need a sound that's going to tear open the mix and rip it to shreds, I'll reach for Omnisphere again. Yes, the other three can do this, but IMO Omnisphere has a more compelling, rich capability of noise. There's just something that sits right about it. [shrug]
The second type of sound is, obviously, evolving pads. It's full of them, having grown out of Atmosphere. They're all incredibly usable, and--honestly--with all of the mod routings, you'll never run out of variations if you're a bold programmer. When I need a pad, the first synth I reach for is Omnisphere. Period. If you write pad-heavy ambient or the like, buy Omnisphere. You'll love it.
That said... :D ...I own and use both Absynth and Massive. I don't understand WHY (because of Omni's great mods, this shouldn't be the case), but Absynth is STILL better at "weird shit" that evolves dramatically over long periods of time. That said, Absynth lacks the ability to make you say "whoa" at the width and power of the sound being played. THAT said, the two of them stack very very well. ...And Massive has better real-time control, period. Being able to set real-time knobs to MULTIPLE destinations with custom depths makes it a keeper. It sounds very digital to my ear, but in a good way. The pads from massive are still some of my favorite pads ever, particularly when performed with the knobs.
Also, the IN-ability for Omnisphere to layer (easily) more than two oscs for a pad is a mark against it. Yes, you can set up a multi to get four or six or sixteen oscs... but that's a PITA and will devour CPU. I prefer Absynth and Massive's three (plus) oscs for layering up some complex pads.
The third kind of sound Omnisphere gravitates toward is those LUSCIOUS bells. You'll hear them all over the place, once you own Omnisphere, because THEY ARE AWESOME, and it's very, very hard not to use them. :) I'm addicted.
The fourth kind of sound is harder to pin down... but it's those "weird samples" that are characteristic of the synth, like the burning piano or the choirs or Tesla Coil or...or... well, when/if you own it, you'll see: they stand out. And like the bells, they are very cool... but also they are VERY characteristic. Painfully so... so much that I'm disinclined to use them, because they're the kind of sound really stands out, and everyone will be either saying "Whoa! Cool!" or "Totally copied from _this other song they heard that used the same sound_." [shrug] They're nice to have, but I feel like I would never actually use them, so it's a bit wasted. ...Does this make sense?
...Anyway, I stick by my original statement: Omnisphere is a top-five, all-time synth. If you're jazzed about synths for the sake of synths, you really need to own this. But if you're looking for a particular type of sound... listen to the demos. Repeatedly. ...Because you're going to end up sounding a lot like them. ...Don't get me wrong! These are world-class, top-notch sounds. Some of the best ever. ...but they are very... Omnisphere. I think "Omnisphere" is likely to be an adjective that sees more and more use over the years. :D ...Or perhaps "Spectrasonic", since Trilian tends to sound quite similar, too. ...Which leads me to believe (I don't own Trilian and haven't tried it) that it must have a lot to do with the effects. [shrug] ...The effects, BTW, are very, very good. It's like having Guitar Rig attached to your synth.
A few additional specific notes...
The filters on Omnisphere are really, really excellent. Lots (!) of variety and very good sound. "Juicy" in particular is wonderful (I think this is the impOSCar filter, honestly). They are not as good as the filter on Diva... but that's not saying much; nothing comes close. :D
The envelopes on Omnisphere are... good, but a little frustrating. I wish I could get a snappier attack, sometimes, and they seem to jump quickly from short to VERY VERY long... wish there were a little more control on the sliders, there. That said, the complex envelopes are better than (easier to use than) Absynth. Also, as I'm sure others have noted, many sounds come with slow attacks in the sample. This is, of course, easy enough to fix by moving the start time along the sample. Do it.
The "granular" aspect of Omnisphere is weak. I mean, it's useful! And it sounds nice... but it's not *really* granular and it's not very flexible. Ignore this as a feature, really: it's just a way to add a little motion to a pad, mostly.
The "Orb" feature of Omnisphere is INCREDIBLY AWESOME. Watch a video to see what I mean. This really is very, very useful; they cannot oversell it. ;)
Patch-management in Omnisphere is the best I have ever seen, hands-down. No contest. You will grow increasingly frustrated trying to use other synths because they can't handle patches this well. (Native Instruments, are you listening, you bastards?) It's insanely good. Don't underestimate the awesomeness of this feature.
The multi feature of Omnisphere is one of my favorite things about it. I load up one Omnisphere in a song and just patch the channels to other MIDI inputs... I actually don't think I've ever had two instances of Omnisphere in a single song, now that I think of it.
And, of course, Omnisphere is... heavy. It loads slowly, it changes patches slowly, and it has such a HUGE collection that it will take you a while to find what you want (and you will waste at least a week of your life simply going through the patches and rating them, I promise you).
In summary, Omnisphere would be my desert-island synth: I wound't hesitate to choose it. ...But if I were writing music on a desert island (inexplicably wired), my music would definitely smell of Omnisphere. That's probably not a bad thing, but depending on your situation and attitude, it may be something to consider. It is well worth the price tag: no three other VSTs are better-sounding or more capable, over-all.