The WatchKit SDK can seem almost frustratingly limited at times, but I think that's for the better: I'd wager that the first crop of 3rd party Apple Watch apps are going to be a lot more rock-solid than the first crop of 3rd party iPhone apps, due mostly to the focus imposed on us developers by the constraints of the SDK.
I've been working on a watch app(Gaia GPS https://www.gaiagps.com/) and went to one of the labs to test it a few weeks ago. I dont't think I could have shipped a really useable app without testing on actual hardware. There was nothing majorly different from the simulator, just lots of minor things.
I found the API easy to use, it made for quick development due to the limited scope of what can be done. It seemed like if there isn't an obvious way to do something Apple doesn't want it done.
I have some experience building WatchKit Apps. I'm building out the companion App for my virtual trainer:
http://impactfit.co
It's very early days with WatchKit. You can understand most of the SDK by just looking at the header file for WKInterfaceController. A Watch App is basically a Today Widget Extension.
WatchKit is "refreshingly" limiting. I'm building a new iPhone + Watch app from scratch & I have found that the limitations of Watch tend to spill over into the design of the companion IOS app. You start to think in terms of "Notifications" / "Actions" instead of trying to be a "Destination" app. Good times.