I've already switched from the first Galaxy S to the Galaxy S2. I think this particular switch from GS to GS2 is because we all loved the first GS, it's a great phone. So, we couldn't wait to put our hands in the new one, which is even more amazing!
Oddly, I like the GS's design more; so, at first, I felt the GS2 to be a bit huge, fragil and squared (since I had both I could compare them). Now I've sold my GS (and I must say I miss it), and as time goes by, I'm more and more used to my GS2 which is absolutely awesome! =)