This e-ProMagazine article,
Converting From Notes to .NET, According To Microsoft is definately worth a read. It references a document which was released by Microsoft some time ago, that was promptly pulled from circulation. As you read the article, you'll see why fairly quickly. A lot of their claims and "strategies" for migration are completely flawed, and some of the statements they make are flat out wrong.
The article is definately worth a read for anyone who may have to provide a list of pros and cons in order to deter a possible migration, and I suspect that although most of the article is geared towards Notes -> Microsoft / .NET migrations, that a lot of the arguments and points raised will hold true for potential migrations to other platforms as well.