I own the S400 and like it very much as a "take everywhere" complement to my regular camera kit. Very good image quality, excellent build quality, small form factor, reasonably fast shutter release (using prefocus), and very acceptable prints to 8x10 and even 11x14.
But one has to comfortable with the trade-offs: no manual controls (but good exposure compensation), no RAW, somewhat problematical autofocusing in low light. Also, the optical viewfinder is barely adequate at best but this is true of most digitals in this class.
If you want more control, go for the S50.
Phil Askey just posted his review of the S400:http://www.dpreview.com/reviews/canons400/