I too was all about Nintendo back when Dreamcast launched... Even going so far as to claim that the N64 with a RAM pack upgrade looked about as good as the DC! (Yeah right!)
Early on, I played a few minutes of Sonic Adventure at Hollywood Video, the chain that had exclusive demo kiosks in the months leading up to the North American launch. I thought Sonic Adventure sucked, because all I'd played were portions of Station Square (i.e. non action stage parts).
I had no interest in the DC until Xmas 2000, when I got one as a surprise present. I actually wanted a Saturn more at the time to play NiGHTS, Panzer Dragoon, and other classics I'd missed. Initial impressions weren't good when Shenmue and my demo disc wouldn't load, and I called Sega support and gave them a hard time ("You wouldn't see Nintendo systems do this kind of thing!")
But once I fired up Emerald Coast in Sonic Adventure, I really got impressed by the console. SA is still one of my favorite games to this day... I did just about everything in that game. And Shenmue (once it loaded up) really pulled me into an immersive experience unlike any game I played before. Loved it. Future games like PSO, Alien Front Online, NFL 2K1, all affirmed how awesome the Dreamcast was, and cool add-ons like the VGA box, microphone (played all of Seaman), fishing controller, etc. opened my eyes to the talent at Sega.
My newfound enthusiasm for the Dreamcast became bittersweet when Sega announced that it was going software-only just a few weeks after I got my DC in early 2001. The DC really changed my view of videogames, and spelled the end of my "Nintendo above all!" philosophy of gaming... But that's another story.