Seems to be the best choice for .NET applications, but to be honest: if you stray from the default settings you will be in a lot of pain most of the time.

TeamCity supports both MSTest and NUnit (which is open source) to run tests.

TeamCity offers well defined APIs for extending, as well as a REST interface.

The user interface of TeamCity is clear, well thought out and the dashboard is highly customizable.

The user can easily compose dependencies between builds by adding snapshot and artifact dependencies, all on the one screen. All output of upstream builds is available to downstream builds. Triggering sets off the entire build chain and supports re-running of the portions of the chain that failed.

The fact that it is based on Java does not hinder TeamCity's ability to support different build environments. TeamCity in fact supports a large number of languages and tools for each of those languages (build runners and test frameworks).

Some of the languages/platforms that are supported include: Ruby, .NET, Java.

TeamCity has different installation packages for different operating systems. All the user needs to do is download the correct one and run it.