Battery life, despite being tipped as much longer on the new iPhone, seemed to be pretty similar to the iPhone 3G.
When browsing the web, calling and uploading over 3G intensively, we hit around 5-6 hours of functionality, which is pretty much the same as the last iPhone. Over 2G we managed to make the phone last for a day under heavy use, but that's mostly because we got too irritated with the web to really use it hard.
In normal use, with the phone searching for a 3G signal, it works pretty well for around a day, with some video watching and song playback thrown in as well. We haven't tested it solely as a media player, because if that's all you're buying the iPhone 3GS for, then you're better off with a standard iPod touch really.
One thing that did irritate us was the fact that while tethering to a PC or Mac the battery didn't seem to charge at all, despite claims to the contrary. It's only a little niggle, but we hoped that it would be able to survive some data transfer and top up the juice at the same time.
The calendar function of the iPhone is a bit simplistic too - the normal version is somewhat akin to any basic mobile phone. Out of the box it seems there's not a lot you can do, as the calendar doesn't prompt you to sync up with anything.
However, Google has kindly put together some instructions on syncing your calendar (and contacts and Gmail, of course) so if you've got the time you can get all settings-ified and use push to sync with your calendar. There's also the option for syncing with Mobile Me should you want to be all integrated with your Mac, although neither of these options is particulalry well publicised in the Calendar application itself.
We're also seeing the entrance of Voice Memos with the new iPhone 3GS, with the ability to record sound and watch the meter at the bottom twitch in time with your voice. Yes, it looks nice; yes it adds a bit more functionality, but Apple, really? It took you this long to work out how to put it in the iPhone? Shame on you.