How is bangle different from other smart-watches?

Do you mean to develop an Android app or, as an alternative create a webapp that uses Web Bluetooth?

Well, for a lot of the apps (eg GPS recorder) right now we've got the app, and then a webpage which interfaces to it by Web Bluetooth to pull the data off. It's pretty trivial to do that and there are tutorials on it. Or you can do it really easily outside of the app loader as well, just on your own website.

The only issue comes if you want to display or report 'live' information to/from the internet. You basically have to make your own Android app to do that since Web Bluetooth apps can't be run in the background.

I saw the expected battery life is of one week, but is that realistic?

Yes - the firmware they ship with should manage about 1 week as it comes if you're not looking at the screen. It really depends on screen/GPS usage - if you have it set to turn on when you press a button then IMO you should be getting near a week. If you have the screen on full all the time you're only looking at maybe 6 hours!

However on 2v07 (when it's released) or on the 'cutting edge' builds the idle power consumption is massively reduced - you should be looking at around 90 days if left on the side, and maybe around a month with relatively minor usage.