My very first reaction in the theater was confusion and not really knowing exactly what I had just seen. Like Knightmessenger said, it happened fast and didn't make much sense.
Up until that point, I had watched the Original Trilogy enough times to have every frame memorized. I also had spent my life up to that point regarding George Lucas as God, so it would be a long time before my mind was ready to process the possibility that he would do anything less than awesome (this realization was so hard for me that it didn't really sink in until several years after Revenge of the Sith came out, after I had lots of time to meditate on the prequels and special editions).
When I was sitting in that theater in 1997 experiencing the Special Edition for the first time, I went with the expectation that any changes would be done because of special effects that weren't possible in the 70's and 80's before the digital age, and also to make outdated-looking effects shots look better. Since the Greedo scene didn't fall into either category I wasn't expecting anything to be different about it, so the change left me scratching my head.
Now that I've had more time to digest the Greedo change, I have pretty much come to feel the same way about it that most fans do, although for me personally it's not one of the worst things about the Special Edition and I think it might be blown out of proportion a little by the fans. Don't get me wrong though, I am against it; it was wrong of Lucas to do it. It's just unfortunate that Lucas made other changes that were a lot worse (in my opinion).
Have you seen the picture of Lucas wearing a "Han shot first" t-shirt with a grin on his face? That grin annoys the crap out of me. Whether fans are happy or miserable, he gets to have pride over being the guy who gets everybody riled up. What he really deserves is for nobody to care. But then we come back to the fact that he gave us Star Wars in the first place and we wouldn't have it without him, so we can't simply hate him. Damn you Lucas. If I met you, I would want to kiss you, shake your hand, and punch you in the face all at the same time.