Clicking the "Download" button in the Visual Studio Extension Manager or the "Get Now" button on the website takes me to the release page for Wix 3.5 but the current version is 3.7. You may want to link to http://wix.codeplex.com/releases since that URL seems to automagically redirect to the current release.
There are many advantages of using the VSIX format that cannot be achieved by using a general installer technology like .msi's. One if it being able to pass through Visual Studio specific meta-data. If you look at other add-in models, like Firforx, they don't rely on msi's either.
@stewils, the inability for an MSI to pass through "Visual Studio specific meta-data" is an implementation decision by the Visual Studio team. If Visual Studio chose to expose those endpoints to an MSI then everything could work together smoothly. Unfortunately, Visual Studio seems to have decided to create another installation package format.
Also, Firefox is a poor example. That application is designed to run on several platforms. The Windows Installer is specific to the Windows platform. In that light, I can understand their decision to use a cross platform installation mechanism. However, Visual Studio is a Windows only application. Creating a new installation mechanism simply creates more work for developers like those that work on the WiX toolset.
@RobMensching I was just impressed by your reply :) I partially agree with you, Firefox extensions are specific to FireFox Browser platform which has its own app specific features hence no point in going with Windows based installation packages. FireFox (Browser / App) adds another layer to Windows (OS). Similarly Visual Studio a Windows based app has its own features. There could be many such applications like Visual Studio that run on Windows Platform. If we keep modifying the MSI to support app specific meta tags, it could make MSI architecture complex or it may limit Visual Studio like applications with MSI architecture. In my opinion, either are not good. So I believe it is a rite decision made by the VS Team to go for independent app specific package file (VSIX). At least they are not bound to MSI architecture limitations.