We use Forests and they are pretty awful. I would recommend against using that extension. It is awkward to work with, makes it hard to rollback your repos to a consistent state, and is painful when you have a change that applies to both repos.
Aha, perhaps I have found the Mercurial equivalent of git submodules! ;)
Although, actually, I'm pretty sure you can get git projects with submodules to roll back to consistent states. But it might take ten separate commands, five attempts, and a visit to the FAQ. Submodules are conceptually tricky and their workflow is a mess.
Sound exactly like forests. You CAN get back to a consistent state. But you have to be very careful look at the commit history and hg update a few times.
I've used forests on a few projects. On the upside, the commands don't cause unpleasant surprises; it's easy to guess what they'll do. But there are still far too many extra steps to do simple things like clone a remote forest and get all the sub-projects up to date. They need an "hg jfdi" command.
