Synchronizing two CVS repositories using Mercurial

We're in a situation where we have access to some of our partner's source code. We need to keep in sync with their repo, otherwise our copy would quickly become stale.

Currently, we can do this manually because our changes are still small. A few months from now, however, our changes will eventually drift away from their code and merging it with our tree would become difficult.

A distributed version control system is in order. Here's how I did it:

+-----------------+         +------------+                 +------------+         +----------------+
| Upstream        |         | Immutable  |                 | Working    |         | Local          |
| CVS repository  |-------->| Mercurial  |---------------->| Mercurial  |-------->| CVS repository |
+-----------------+ import  | repository | Mercurial pull  | repository | export  +----------------+
                           +------------+                 +------------+
                                                               |^
                                                               ||
                                                               ||
                                                               v|
                                                              User

In theory, exporting it back to CVS is no longer needed. In practice, however, I needed to maintain "compatibility" with my fellow developers who only use CVS. I hope we'll eventually migrate away from CVS though because merging lots of files is really time consuming. Merging/branching is CVS' main weakness.