Perhaps I'm exaggerating a little bit, but if you look at the claims Gallio makes you might be tempted to agree with me. 

The Gallio platform seeks to facilitate the creation of a rich ecosystem of interoperable testing tools...  they should present consistent interfaces to the world so that they can easily be integrated into the systems and processes of the enterprise.  At present Gallio can run tests from MbUnit versions 2 and 3, MSTest, NBehave, NUnit, and xUnit.Net.  Gallio provides tool support and integration with CCNet, MSBuild, NAnt, NCover, Pex, Powershell, Resharper, TestDriven.Net, TypeMock, and Visual Studio Team System.

That's a lot of integrations!

The UI runner that comes with Gallio is pretty and for the most part functional - remember this is an alpha!  I did have some issues when I tried loading several thousand tests into the UI.  Things got slow and a little wonky while it scanned for tests in my assembly. A couple of times it crashed on on me after reloading the assembly after a recompilation. Then again I'm not sure if NUnit is any better in this type of scenario.

Gallio sounds like a real benefit for organizations using MSTest that are looking to upgrade to a real xUnit framework.  At the very least you can ditch the lackluster MSTest runner in favor of a better runner like ReSharper, which is what I'm currently trying to do.  Unfortunately there's an issue with Gallio that makes all the tests report back a status of 'skipped.'  Luckily this is an OSS project with public source code...

Gallio makes working with different open source projects very easy from my point of view.  Now I can use a common interface (ReSharper 4.0 test runner) for all my testing needs regardless of the OSS project's test runner... no need to fire up MBUnit GUI or NUnit or whatever.  It all just works and doesn't interrupt my flow.  I imagine the number of testing frameworks and tools for Gallio will almost certainly grow over time. 

This is definitely one of the more useful testing tools I've seen recently.  Why?  Because it reduces friction.