I don't have Logic 9 yet, but I can definitely recommend Omnisphere. It's the most inspiring instrument I have seen or heard for many years. The sounds are awesome and the user interface is superb. Endless possibilities.....
And no, I don't work for Spectrasonics.
I can agree on all the above: amazing plugin, new approach, very CPU and RAM intensive.
BUT, it still has bugs with Logic.
The PlugIn Preset in Logic is not fully supported. and results in weird issues.
When you save a sound with Logic's PlugIn Preset , Omnisphere will turn on stacks and the first four parts for no apparent reasons
Spectrasonics is so convinced about the greatness of their build in sound management that it seems that they don't care much to make sure that it plays well with Logic's great sound management system.
It got to the point where I caught Omnisphere "red handed", when I opened a project from a few days ago and it sounded different. When I compared it with the mix and the bounced tracks I found that the sound had actually changed or not saved properly.
So better right your patch names down (Track Notes in L9 not a moment too soon) and keep a close eye on it, especially in complex mix situations.