The current HALion 3 release (3.1 final) is stable in every regard; a _huge_ number of bugs has been fixed since the very first build of 3.0. The integration in Cubase SX has always been great and very straight forward. I don't think the user interface is very beautiful, but it does what it should. The interface has been improved quite a lot for those people who create or edit programs. The new effects are really fine.
The filters are much better since version 2.x, and the delivered content is very usable and of high quality. The drums and strings are simply great (the drums already were in 2.x). BUT: The delivered content is preset only; you can't extract the samples or use them to create own patches, though you can edit the programs. So if you are used to archive your songs together with the samples you use, e. g. on DVD-R, you won't be able to, except you burn down the complete content archives of HALion 3 again and again (4 huge files).
With 3.1 (final) a new streaming mode was introduced, that gives a lot better performance than any previous HALion version. Comparisons with Kontakt show that it beats the latest version of the NI sampler, too.
HALion 3.1 finally is a mature product and is recommended to anyone who needs many many voices, excellent disk streaming and a powerful and really complete sampler.
HALion 3 comes together with a front end so it can also be used stand alone. According to the forums, there's some issues left with this and certain audio cards, and preferences are not always remembered correctly, so I recommend to use brainspawn forte or a similar host instead.
There's a quite active support forum at http://forum.cubase.net.