It depends how you work and how stable your product is. I think they are patch heavy i.e the CVS tree is used to apply patches to and that is that. CVS has some advantages then as you can pin individual file versions on a tag rather than having to fuck around merging things. Individual files might be ethernet drivers, scheduler etc because TBH the BSD codebase is a hell of a lot better decoupled and modular than most things out there.

CVS works because they didn't fuck it up.