I find it funny that the Nexus phones are iPhone Killers. And yet, how many Nexus S phones have sold? My guess is fewer than the iPhone 4S sold on its first weekend. Samsung reported that ALL Galaxy S 1 phones (of which the Nexus S is part of) have sold 20 million units. I highly doubt the Nexus S accounts for 1/4 of them.
The Galaxy Nexus looks like a very solid phone, but based on what I've seen with benchmarks on other similarly specified phones, the 4S should be slightly faster in regular processing tasks, MASSIVELY faster in graphics performance (they're using an old GPU that is nowhere near as good as the one in the 4S) and will likely have worse battery life. Yes, the screen is much bigger (too big IMO, but that's for each user to decide), and very high res, and that's awesome. ICS looks quite nice (they stole some good things from webOS). Camera is meh.
Hardly an iPhone killer, unless you base your purchasing decisions solely on screen size, and you want huge.