Bazaar is an excellent open source version control system, which allows you to track project history
What is most valuable?
A great tool to work on projects in a collaborative environment.It also provides an ability to commit work on local machines, in case of no internet connection.
What needs improvement?
Sometime it takes too long to display the differences in working tree, between revisions.
Several operations are tricky and confusing (Like criss-cross merge).
*Disclosure: I am a real user, and this review is based on my own experience and opinions.
