I just got my SGS II 2 days ago. So far from what i can conclude after 2 days of use :

1. It produce more heat that SGS.
2. Screen is much much much better than SGS, image and text appear sharper. It may be just me, but comparison with my old SGS, this new SGS 2, contrast is much better than SGS.
3. Boot up and shutdown time of SGS 2 is much faster, logo animation during bootup is much smoother, still not lag free, but compared to SGS it is much better. Shutdown is also faster, it is now almost instant after the shutdown animation is done.
4. Browsing with SGS 2, is much much faster.
5. GPS is accurate and reliable compared to old SGS. I was sitting outside at a mamak on Saturday night, and running google map with SGS 2, it show me right where i am, with no sensor aiding. Just using GPS satellite options.

All in all, it is an improvement over SGS. Not by a lot, but noticeable difference. If you are the type whom thinks the old SGS is something quite sluggish, an upgrade to SGS 2 will pretty much satisfy you.