Migrate Objects Between Forests

I need advice on migrating objects between AD forests. My company will be using this to make a decision on our AD structure.

I know MS has a migration tool but I need to know how well it works, how much time it takes to migrate a server, any pitfalls, etc. The white papers I have seen all talk about migrating from NT to AD but not much about moving between forests.

Any ideas from experience, or sources of a good white paper/book would be appreciated. I need proof to make my case that management wants too many forests in their design.