As activity monitor - the watch uses accelerometer and heart rate monitor, and the iPhone's GPS. That should make it more capable than any other activity monitor. The Garmin Vivofit, up to now tied for the best monitor, requires you to wear a HR chest strap, and doesn't have position/speed data nor get it from another Garmin device.
AFAIK. The Bodymedia Core, previously the most accurate calorie burn measurement tool, doesn't get heart rate or position/speed.
As smartwatch - it is a wrist UI to the apps on your iPhone, much like other smartwatches. The dial interface is interesting. It will really help if Siri has been improved. The watch will have Apple's Pay functionality, which no other smartwatch will have. I didn't catch anything about water resistance, which is a big deal for me.
$349 seems an accessible price, given Apple's customer segment. But not available until early 2015.
Edit: Uh-oh, there was no mention of battery life . . . if the Watch has to be charged nightly, then it is a non-starter.