- It would be really nice if you could branch from an SVN or other semi-sane system, do multiple commits, then submit your changes back to the mainline using the 'submit' command. Doing this with CVS would probably be an exercise in misery, but any SCM with atomic commits could be a target. We could ensure that all imports of the same source produced the same data, so that parallel imports could merge from each other.
I think ForeignBranches have killer ease of use. The qualitative difference between working on an imported branch and just bzr branching it is huge.
There is a plugin that adds support for foreign Subversion branches. It allows most Bazaar operations to work against Subversion branches, including merge, push, pull, commit and uncommit. Includes SVK support.
There is a plugin that adds support for foreign Git branches. It supports pull from local and remote git branches and support dpushing into git branches from Bazaar.
There is a plugin that adds support for foreign Mercurial branches. It supports pull from local and remote mercurial branches.
There is no support for foreign Darcs branches yet, though this should be very well doable. There needs to be some care because of what Darcs considers to be idempotent versus what Bazaar would consider the same. (Darcs treats a branch as a set of revisions, while Bazaar considers it a history of revisions. So in Darcs the histories (A, B, C, D) and (A, C, B, D) are considered equivalent, it just means Bazaar needs to be careful with what it considers ancestry for a Darcs branch).
There is a baz-import plugin that can convert Baz repositories to Bazaar repositories.
There is no support for foreign CVS branches at. Creating it may also prove hard as CVS lacks atomic commits or unique identifiers to identify branches.
However, there are options for converting from CVS without foreign branches:
https://launchpad.net offers a CVS repository tracking service. This is great for tracking a branch that is in CVS.
There is no support for foreign Monotone branches yet, but it should in theory be possible.