Hi,
I'm using Emacs for mediawiki editing on a daily basis. mediawiki-mode has a number of cool features including syntax highlighting, snippets, quick moves, helpers for talk pages, ability to open, navigate and publish without leaving the editor… And as it's not an editor designed specifically for wikitext, you also have the extra benefit other modes such as javascript or css.
There is no contextual auto-completion, though; ie, completion is only based on what has already been typed before.
Best regards,