While Subversion is for many the source code revision system of choice, I've never stopped long with it myself. Why? Because it's mostly a fixed-up CVS, and doesn't fix the really gnarly problem with revision control: merging.
Mark Shuttleworth has written an insightful little article on the key aspects of merging source code. Some of the points he makes underpin my choice of revision control system.
While most articles praising Bazaar are reports of open source development, I've also found it very handy for small in-house teams working on web applications.
There are very often multiple arcs of development going on at once, and the ease of merging makes it easy to ensure that long-running development arcs don't get ridiculously far from the main trunk of development. Bazaar's merging also makes it easier for lead developers to police the merging of the code base.
Working mostly with Rails, one repeatedly runs up against the assumption that Subversion is the golden path for revision control. This is perhaps a little telling about the level of participation most Rails-based projects have reached.
The tools story for a Bazaar user is both good and bad.