It wasn't always that easy. Integrating Office apps prior to the introduction of VSTO was ugly and fraught with incompatible types. Especially if you were using a language other than VB.NET, which generally meant a whole lot of Type.Missing argument entries in every other line of code in addition to creating marginally machine-to-machine compatible COM interop wrappers. Getting beyond the "Works on My Machine" certification wasn't always an easy thing in VS 2003/Framework 1.x.

I never had much of a problem to be honest and I built some behemoths which involved ASP.Net, .Net CF, remoting and all sorts of other junk in the same app.