I plan on updating my iPhone 4S to iOS 6 but I am going to wait a couple of weeks and wait for the initial Oh-my-gosh-iOS-6-trashed-my-phone-and-killed-my-battery-life-erased-all-my-musi c-and-ruined-my-marriage posts to die down.
The maps app will still give directions on the iPhone 4 it simply wont give you audible turn by turn instructions voice by Siri. The directions will be text based like they are now. Apparently the processor in the iPhone 4 isn't powerful enough to run this feature.
iOS 6 looks really good, but it will have different levels of what it can do based on the hardware it is installed on. If I remember correctly all the bells and whistles will be available on the 5 and 4S, things like the turn by turn directions won't be available on the 4 and probably the other Siri functions that the newer phones are getting. The 3GS will load iOS 6 but my guess is that it will run kind of slow, the same way the 3G did when you loaded iOS 4.x on it or even tried to load up 5.x. The hardware just isn't sufficient to run the software. This is why Apple will stop supporting the 3GS now. Keeping up with 3 different hardware platforms is tough enough, I can't imagine trying to keep the current software running on old hardware.