Some thoughts about Mono: It competes with Microsoft in the only areas in which they are vulnerable -technical competency, rate of development, cost, and
possibly technical support. In the real world, I suspect
that its major advantage for Ximian will be as a way to
secure extra funding by associating themselves with this
quarter’s big thing.

Admittedly, I haven’t bothered to investigate .NET, but
after
in-depth experience of development with Microsoft tools and
APIs in the past, I have very little confidence in the
technical merits of .NET. And having followed tech-news for
many years, it is clear to me that MS uses business
relationships to gradually make other companies critically
dependent on them, and then withdraws cooperation as soon as
it is clear that doing so would destroy the other company,
or at least destroy the project.