Intuit uses MediaWiki in the majority of its internal wikis - over 100. We have one MediaWiki running with fck. Some groups are migrating to Jotspot since it provides wysiwyg.
The lack of wysiwyg is indeed a hurdle for contributors. Even at a "techie" company like Intuit, half the potential contributors see wikitext as a hurdle.
This number likely reflects the rest of the world - it wouldn't surprise me if many potential Wikipedia contributors are also stymied by wikitext. Do you have the number of Edit clicks that didn't then result in a Save? That might be a decent indicator of first-time contribution attempts that failed for some reason - possibly intimidated by wikitext.
The one wiki we have with fck was an add-on to an existing wiki and encountered the compatibility issues Brion mentioned. However, just as the community cleans up wikitext pages, the community cleaned up the conversion issues - just as they would in Wikipedia.
You get the closest conversion that you can and then people will clean up what remains.
- MHart