We have been working with the Mozilla team, to see if they can use Bazaar for their development.
We have been having some difficulties scaling up to such a large project (approx 200,000 revisions, 55,000 files.)
However, we have been in the process of tweaking our conversion utilities, as well as the internals of Bazaar to make it faster. We also have been able to exploit one of the semi-controversial features of bzr (file-ids) to allow conversion of pieces of the project, and still have it integrate as a whole.
By breaking up the Mozilla tree into a bunch of sub-modules, we were able to improve our conversion speed to around 4-5 revisions/second. Which lets us convert 210,000 revisions in under 14 hours. Woot!! (a whole-tree conversion before we spent a lot of time was running at about 0.02 revs/s (50s/rev) and was taking almost 2 months to convert).
To bring these separate trees back into a single tree, I wrote a plugin which simplifies merging projects together (especially when you want to merge one into a subdirectory of the other).
Basically it provides a new command:
bzr merge-into ../other/project subdir
Which will merge the 'other/project' tree into the local tree rooted at 'subdir'. After doing this, merging back and forth between other/project and this one is greatly simplified, since bzr records all of the information it needs to know which files should be mapped to which ones.
It even works if you end up moving the files around in the new destination.