Capability

* Number of wiki users at your organization
The highest number of wiki users I am aware of is more than 20 000 on Mandriva and more than 10 000 on Curriki.

* Number of maximum concurrent users
I am not sure of the data for this one. There is probably some data somewhere for XWiki.org but I do not know who has access to it.

Wiki Engine reliability (on a scale of 1 to 5, 5 is best and 1 is worst)

I'd probably give this a 4 or a 5, but I'm not sure of the meaning of the question. If you meant "does the wiki engine behave randomly or erratically", I'd say that with the coming 1.0 release the engine is reliable, so that's a 4 or a 5 when used with standard configuration (Apache Tomcat, supported database and so on...)

Usability

* Wiki Engine usability (on a scale of 1 to 5, 5 is best and 1 is worst)
There is some work going on right now on the improvement of the standard wiki usability. Its is currently at about 3, with the aim of reaching a 5 soon :-) However you should be aware that since you can write your own skin and completely customze the look of your XWiki usability may be better / worst on your customized installation.

* Training / learning curve (on a scale of 1 to 5, 5 is easiest and 1 is
most difficult)

Depend on what you want to do, as a user or as a developer. As an user, with the WYSIWYG editor using XWIki is as difficult as clicking on the "edit" button, so the learning curve is fast. As a developer, it depends on what you want to achieve but the community resources and the content on XWiki.org are a good help. Once you grasped XWiki core concepts the learning curve gets faster.

Cost

* Hardware requirements
Depends on whether you want to start with a hosted version or do the hosting yourself. You'll probably need a server :-)

* Cost (license fee, hosting fee etc)

The cost depend on the license you want to use. If you are using the Open-Source license, it will cost nothing in itself and you will have internal development costs to achieve the XWiki you want (with specific applications, skin, etc...).

You can also use a packaged version of XWiki (that is, fully ready to use), you should contact pascal@xwiki.com to get an idea of the offering and pricing. In short depending on the size of your organization you can use a wiki or a wiki farm with a setup cost and a yearly subscription fee.

Support

* Data backup and recovery (i.e. do you have daily back up procedures in
place and is it difficult to backup and recover the data if there is a
disaster?)
(can you restore just a page or space instead of the whole wiki if a
page/space is messed up?)

I think Vincent is operating a regular backup on XWiki.org. This is made easy with XWiki thanks to the Export feature which lets you export all the content of your XWiki instance and store it in a XAR file (you can read it with any ZIP software). If you want to import back a specific space or page, you can do it by modifying the XAR file so that it will contain only the files you want to bring back. In short, data backup is easy, 5.

* How difficult is the administration? (i.e. roughly how many hours the
administrator need to spend on the wiki maintenance?)
This will probably depend on the number of users you have. Do you mean only technical maintenance or reversing users random changes too ? Tools such as the what's new, the customized RSS feeds, the panels application and so on make the administration easy enough but it could be improved. Maybe a 3.5 or 4...

* How is the quality of support of this wiki engine (from the commercial
vendor or the community. i.e. can you get timely answers for your
questions?) (on a scale of 1 to 5, 5 is best and 1 is worst)

The community support is really good, as you will know if you are a regular user of this mailing list. On the commercial side, With each XWiki offering comes a support contract that will provide you with a guaranteed response time, less than 1 day, maybe even less, check with Pascal. A 4.5 (for support is never good enough ;-)