It shines a destroys the competition in everything ambiance related, it has so many pads, very complicated soundscape patches, with a lot of character and which sound 3D right away. It has a lot of very original sounds too, and their team has been quite creative at creating sounds out of anything (I think one of the source sounds they use is a piano they put on fire or something). It has everything you need including bass and lead sounds as well, in fact I played with it to create electro tracks with it exclusively. The only thing is that it' is more complicated to do your own sounds than more traditional synths because of its complicated GUI (I don't think it is as friendly as an FM8 or Massive). In theory you can do almost everything you want with it, but in practice to create a sound from scratch out of oscillators I would go with a simpler synth, and you're better off starting from a genuine patch that you would modify thereafter. Also it's more based on granular sounds or samples of actual recordings modified to the extreme (like creating a whole synth soundscape out of a single string instrument sample), which have different flavours, than sounds generated by oscillators (which are richer in harmonics).
Basically you have thousands and thousands of sounds so if I had to keep one single plugin for all my synths, I would go with omnisphere.
Its drawback is that most of its good sounds are a bit "too 3D" to be used in the background, so if that's what you are planning to do, it's good to downsize the sounds by removing their reverb or internal omnisphere plugins IMO (most of them have a shitload of compression / delay / reverb included). Of course in the thousands of sounds, you have hundreds and thousands of simple sounds too, so it's worth a try in any case. What I did myself is go through all of them, playing a few notes, and giving a rating inside the omnisphere explorer to all those I found inspiring, so that later I could filter the list and only see those I found potential in.
Some of them are very nice but their attack is so slow it's not usable except as a soft and warm pad type of string sound. In fact the attack is so slow it's late when you get a chord change in your song and it gets distracting, if you're trying to use them in the background of your guitars to make a wall of soubd. I didn't try to edit too much the patches though. I know most of them are based on the same samples (for string sounds) (you just need to read the description of those patches when you load them) so I found most of them shared this characteristics. I would have expected more possibilities for strings. Mileage may vary, I found out it's all about experimenting the patches, sometimes trying them in odd octaves.
