I've got the Galaxy S. While it is a good smartphone, there are many do'h instances where it would've made this a great smartphone.

The biggest complaints:
1) Build quality is superb - but its superb in plastic. I only dreamed for a HTC Legend type housing.
2) Software quality is atrocious. The froyo update fixed my GPS and gave me a free (offline) Navigation software. But it still has that darn stutter/lag, the memory management is mediocre at best and there are a few glitches. (Also the interface is worse than before). I am trying to be patient, waiting for a MIUI build!!
3) Buttons are bad. While I appreciate having a physical Home button, the quality used for it is suprisingly low. The have placed the Back button on the right instead of the left (like anyone with half a brain would do). Putting the power button on the right side is such a frustration its difficut to lock the screen with one-hand/busy. Putting the volume rocker on the left side (copying iPhone?) also means I need to use my left hand and I always asumed that people would want to hold it with their right (right hand dominancy in society). And finally the size of the rocker/button is quite small so it is difficult to hit it, especially if you have a case for it.
4) No LED flash. Which means no pictures/videos in dark areas. Which means you have no flashlight on you when you need to see where/what you're doing. Both instances I've encountered before and it left me feeling angry and depressed.

Because of these issues, I've lost some hope in Samsung (they were my second favourite after Nokia). So I will skip the Nexus S, I just don't see how it is an upgrade from the Galaxy S. But when it comes out I will totally; sell the SGS, add some money on top, buy an unlocked/noplan PSPhone, use my current 2yr plan.