Enormously flexible, but starting to show it's age a little now. Has great potential but missing some cutting edge features that are already incorporated into other Steinberg instruments, for instance the resynthesis engine from Backbone, the ability to split noise and tonal componants from Groove Agent, and then there's other aspects that I'd like to see that would make life easier within the UI, drag and drop modulation sources that are starting to become norm in many instruments would be really useful too.
If you're after soundpacks then Halion is pretty lacking in this area, Kontakt has pretty much cornered the market, so if that's what you;re looking for then I'd go with that. But if you want a powerful instrument that can handle all types of synthesis as well as being a solid sampler, with awesome integration for Cubase then this is a really good option. It can handle opening some older format Kontakt libraries, but I've not had much luck with much of these.