Since I've got an iPad 2 (which is obviously the most powerful iOS device by quite some way), the enhanced performance of the 4S doesn't really appeal to me. I play all my games on the iPad, for example.
The camera, although it sounds cool, doesn't seem that much better than the 4's. I've been looking at sample photos on Flickr and elsewhere.
Siri is pretty cool, but I've tried it on my flatmate's 4S and it seems like something I wouldn't use very much, except out of boredom or maybe in the car.
In summary: I'm happy with my iPhone 4-iPad 2 combo and I'm not willing to spend approximately £200-250 upgrading my iPhone for these features. The 4S is a fantastic phone and would be great for 3GS owners or those new to the iPhone, but I don't feel the need to upgrade from my 4 yet. I'm gonna wait for the redesign (which is very likely to be the next generation).