I'm moving on from a 3GS so I plan on getting the 4S.
In the face of all the rumors of possible updates, an S version of what we already had is underwhelming. In the past Apple has done a great job of strategically leaking info to the press when rumors were getting out of hand (iPad 2 Retina Display is a recent example) - that did not happen this time.
It's certainly possible this set of improvements adds up to a terrific phone. But the ad did nothing at all to sell that to me.
In the end, you have to evaluate the device on its own merits. The iPhone 4 may be Apple's most successful product ever, so perhaps this is just "if it ain't broke"-ism. The iPhone 4 is holding its own against all the Android competitors; is it the C.W. now that the 4S will do worse?