My two cents worth as a wiki content creator and super-user, rather than an administrator or developer:
I am currently editing a document in Sharepoint Wiki as I type this, and it is by far the worst editor I have ever come across. To be precise, I'm using Sharepoint Foundation 2010 (previously known as WSS), editing pages using IE 9.
To sum up the problems I've faced: When creating wiki content you want to concentrate on the content and the wiki engine should be so easy to use as to be almost invisible. With Sharepoint that is not the case. I really struggle with the pseudo-WYSIWYG editor, having to fix frequent formatting problems.
I estimate that I'm about 15% less productive writing wiki content with Sharepoint than I am with ScrewTurn or Wikimedia because I have to deal with the formatting issues. If I spend a day writing wiki pages I would lose about an hour trying to fix formatting issues.
For background: I've created four internal wikis at our company - the first in Wikimedia, the wiki engine behind Wikipedia, the next two in ScrewTurn, and a final one in Sharepoint. In each wiki I've written about 50-100 pages.
In both ScrewTurn and Wikimedia the editor looks fairly primitive - a plain text editor that uses simple wiki mark-up codes for formatting. Each has a row of buttons that can apply mark-up codes for simple things like bold and italic formatting, and to create links, so beginners do not need to learn the mark-up codes by heart. While the editors look plain they turn out to be really simple to use, especially for fixing formatting problems.
Sharepoint Wiki, on the other hand, looks slick but is terrible for editing. Instead of using a plain text editor with wiki mark-up it has a WYSIWYG editor that looks much more sophisticated than other wiki editors. However it has personality, an evil one. It frequently adds blank lines or changes the colour of text. When I select text to format then go to the Markup Styles dropdown to format it, sometimes the act of selecting an item from the dropdown list deselects the selected text so the formatting applies to text at a random location. Inserting text copied from Word sometimes causes the editor to double or triple up on blank lines between paragraphs at other places on the page. There appears to be no easy way of creating a table, apart from writing HTML.
The biggest problem about the editor, however, is that you can't easily see what is going on behind the scenes so it's difficult to fix it. Yes, it's possible to edit the page's HTML but that really defeats the purpose of a wiki.
The overall impression I get as a user, is that this is alpha-level code that has been knocked up by a summer intern. I know Foundation is the free version so perhaps I get what we've paid for but I cannot believe a professional software company put out this product.