be a little weary of the 3rd party batteries. the only for my s330, now s400 (many of the s compact/ultra compacts use the same battery, except the latest latest one that uses XD mem). anyway, my 3rd party battery was great for about a year. I'm not sure when it went bad exactly, but i just took about 10 shots (w/ LCD, all flash), and it died on me. this is after it was fully charged. pop in the factory battery, and it's still going (i took random shots, video, lots more than 10 just to try to drain the battery, and it's not fresh).
don't get the s400 unless you're looking for ultra portabilty, go for the s50 or similar. i've loved the canon camera's I've used. just wish sometimes you could use AA's as it's a little cheaper.. but hey, gotta give something up for ultra portability.