DITA and Help Authoring
We use Help Authoring Tools (HAT) to provide Help for DITA Users.
We used Madcap Flare 2.5 to produce WebHelp files for DITA Users support:
See www.ditausers.org/help/
We used Adobe RoboHelp 6 to convert the DITA Language Specification files to a Help interface with excellent full-text search:
See www.ditausers.org/oasis/ (HTML version)
and www.ditausers.org/oasis/flash/ (Flash version)
And we have used DITA to create Help files.
We created a tri-pane DITA Infocenter using Eclipse Help that includes the DITA Language Specification, Architectural Specification, and OT User Guide in one searchable interface with TOC, Index, and Search. The DITA Infocenter is widely used as the go-to reference for DITA specs.

DITA Infocenter

But how good is DITA as a HAT?
At this time, for web help the DITA Open Toolkit is limited to Microsoft HTML Help. And HTML Help is limited to Internet Explorer.
DITA OT can output JavaHelp and Eclipse Help. But JavaHelp appears to be in decline.
Since DITA Users is entirely web-based, we want Help files that can play on the web, and on all browsers, not just IE. So MS HTML Help is not good enough. Moreover, we cannot generate MS HTML Help with our web-based DITA Open Toolkit, since we are running on a Linux Apache server.
WebHelp versions from Adobe RoboHelp and Madcap Flare are playable in all browsers, as are many other HAT tools.
Here's a comparison of some leading Help Authoring Tools with the current DITA Help capabilities. See Char James-Tanny's HAT Matrix for many more.
Help format Adobe RoboHelp Madcap Flare DITA OT
MS HTML Help yes yes yes
WebHelp yes yes no
JavaHelp no no yes
Eclipse Help no no yes
FlashHelp yes no no
Members of DITA Users can now build their DITA projects as Eclipse Help. Their output files appear as a Help book in the DITA Users Infocenter.
Help authoring tools have always been topic-oriented. Their topics have generally been like DITA tasks, the steps needed to solve some problem. So DITA should be a fine authoring tool for Help. The OASIS DITA Technical Committee authorized a subcommittee to study DITA and Help, under the leadership of Tony Self and Stan Doherty.
References: