Microsoft provides the watered-down Visual Studio Express Editions for free. These are single-language versions of Visual Studio with limited functionality. They still work for creating very professional .NET applications.
So when is it time to upgrade to the full version of Visual Studio? Here is an overview of simple features that are not included in Express Editions. These are the kinds of features a programmer working in Express would look for to upgrade.
Perhaps the most obvious improvement is the multiple-programming-language support. Unlike the Express Editions, that require a separate installation for each programming language, Visual Studio comes support for VB, C#, C++, and ASP.NET.
Visual Studio comes with built-in support to create MSI setup files. Deployment projects are easy to integrate to application projects and are very customizable. For those developing professional software it is definitely very convenient.
Upgrading to Visual Studio 2008 does not mean upgrading to .NET Framework 3.5. Visual Studio makes it extremely easy to develop for a specific .NET Framework version. Project templates, controls, assemblies, etc are all adjusted to be compatible with your targeted version of the .NET Framework.
At any point during development you can also change the targeted Framework.
Visual Studio 2008 has support for developing native 64-bit applications. Depending on your target market this could be a crucial reason to upgrade.
The full version of Visual Studio comes with greater customization support. Add-Ins, or extensions, which are painfully missing in the Express editions, are fully supported in Visual Studio. Meaning products like RedGate's ANTS Profiler, JetBrain's ReSharper, and Whole Tomato's Visual Assist X can be integrated right into the developing environment.
While the Express editions have a lot of potential, upgrading to the full Visual Studio become crucial at some point. Of course, for large applications it is a given. But even medium-sized .NET applications can require the extensive power of Visual Studio.