I own a Powershot S400 as well and couldn't be more impressed with the battery.
Normally it will last to fill my entire 256mb card (w/ heavy image browsing, lcd use, and flash).
Take away the flash, lcd, etc. and I have taken over 400 shots on one battery.
Nonetheless I have bought a second, just because everyone is always like "Hey I want to see the picture you just took! Please!" And inevitably the camera gets passed around for an hour with people looking at it. Even then I hardly ever need my second battery.
And I definatley agree, Great camera =)
Edit: In light of the last post, both batteries I own are official Canon products.