In general, I would never recommend using software in a production environment that is not marked as stable . I am using version 1.5 and it works pretty well for me.
A few weeks ago, I tested it against the http://ha.ckers.org/xss.html templates and all XSS attacks were identified.
Update
Version 4 of the AntiXSS library is now unavailable (part of the Web Protection Library ), and it is marked as stable and that I am using at the moment until a new stable release arrives.
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