We have a team in Liverpool supporting the company's direct sales motor insurance system, Vision, which is written mainly in Smalltalk.
We first used XP on the project to put Vision onto the Web. This worked very well, though we did cheat slightly by being our own on-site customer. We figured that we knew as much about what an on-line customer might want as anyone else in the company at the time.
We have a lots of issues with on-site customer. A typical project has several customers, none closer then 100 miles away. We also have issues with coordinating multiple development teams, often on different sites, or using cumbersome mainframe development tools.
Nevertheless, we're now using XP on other projects, with good success.
Also, we have set up our own internal Wiki. We thought we'd try using it for technical documentation, rather than WP documents, and its working rather well. The hyperlink capability of a Wiki is good for structuring the documentation, and its makes the documentation easier to write. While explaining something you can throw in links to other things that need describing without disturbing the flow of the original explanation. Then you can go back and fill in the details on the links you've put in.
To see Vision on the web go to: http://www.morethan.com and click 'car', then 'car insurance' then 'car quote'