This is my main source of instruments whenever i feel lazy or in need of a bit orchestration. Now granted, orchestral quality isn't like some pre-recorded sample libraries, but what it has is good enough to be used. It is great library of sounds to have offering large choices for any composer whom work is more about composition.
and though I did explore it when I first bought the keyboard I haven't really used it very much so I can't say much except it sounded great! I probably should investigate the AIR programs that came with the Advance 49 but always end up sidetracked to Kontakt libraries.
with the sheer number of banks on offer there's bound to be a reason to use xpand!2, but a huge number of the sounds i've demoed disappoint me. if your goal is not necessarily realism, you'll find use out of all of the libraries; especially with how many different timbres there are to each instrument. however, at $100 normally it's a hard sell for me. it comes with a number of midi controllers that sell for a fraction of that price, and is a great addition to those bundles, but as a standalone vst you'd be better spending that money elsewhere.
2500+ total presets and parts. Mostly I use Soft Pads, Bright Pads, Strings, Mallets. In my opinion, you don't need Nexus 2 if you have Xpand!2 because it is good enough to make music with this amazing plugin.