The powerful tools within Kiln; and the ability to oversee massive projects, and the detailed tools to investigate changesets and history are some of the best features.
But Kiln's cross-compatibility with FogBugz and Trello are absolutely amazing, and probably the best feature; it helps to tie together the project management and client communication sides of our version control.
Kiln's user interface often seems a bit dated to me, and sometimes it can be difficult to compare two particular changesets if any two specific changes need to be compared, you usually have to track down the revision ID.
Kiln is very useful, and effective in what it aims to achieve; even better if you already use Trello or FogBugz (if you do not, please investigate those products in addition to Kiln)
Kiln is quite easy to use; it is very effective for team collaboration, and the repository and branch management is easy to visualize with the web interface and branch visualization.
Kiln is absolutely essential to our business.
We have massive projects, which requires us to sometimes come back to projects that have not been touched for months; or to investigate bugs caused by changes made months ago.
Kiln's tools allow us to crawl through every change, to compare many changesets, and to easily view the version history of a file over multiple changes.
What's more, the ability to see the changes of some binary files, such as images, has proven very useful in various design changes; especially when the commit messages have been vague.