Be very careful using C# within a Microsoft Installer based installation(like those generated using WiX). By doing so, you place an additional dependency on the .NET framework, and has been discussed many times this is a *bad thing*. Ideally you should choose something (e.g. C++) that can be built to have minimal (ideally no) external dependencies.
I couldn't disagree with this poster more. Adding the .NET framework as an install time dependency is not automatically and blanketly a "bad thing". It is increasingly becoming an "inevitable thing". After all, how else would you publish assemblies to the GAC? How else would you PreJIT assemblies?
I would agree that it's a bad thing to do things like use Installer Classes to use the framework to install services when MSI is natively capable of doing that, but not just because it's a MC CA, but because it's an unneeded CA in general.
The reality is we are now starting to see new API's that have no unmanaged counterpart. We are having developers write managed code and they are now business requirements for doing things that can only be done in .NET or are done in with 95% less effort in .NET.
We can bury our heads in the sand and repeat the '.NET is Evil' group think, or we can get with 2007 and start using .NET to our advantage and demand that the Windows Installer team do the same. Otherwise MSI's days are numbered.... remember after all that only about 50% of the setup space has switched to MSI. Other technologies are still out there.