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When the original .NET Framework (which ASP.NET runs off of) was released in February 2002, some of my colleagues were skeptical and wrote the Framework off as a passing "fad" and chose to stick with Classic ASP. I would later be accused of "drinking the Kool-Aid" by said colleagues because I was eager to begin playing with ASP.NET in all its glory! Looking back on this, I can't say that I blame them; we all had become very intimate with ASP, and all of the web sites that we maintained ran on an ASP Framework. Nothing was broke, and my colleagues saw no reason to fix it. While this ideology of not upgrading based on the initial release of a new technology made sense, it began to hold less water with me with each subsequent release of the Framework.
Eventually web sites become dated, new functionality is introduced or matures, and so on, and it is only a matter of time before your client is calling and saying that he wants a new web site that employs all of this "cool" new stuff. The argument could be made that you could develop an entire "Web 2.0" web site that incorporates all of this new functionality without making the switch to ASP.NET, and I would agree with that, but I feel that you will ultimately deliver a much more stable product (and probably on a shorter time line) to your client by making the switch. In the next section, you examine the differences between these two technologies; hopefully, I make a good enough case to convince you to "drink the Kool-Aid" too!