How much use can you get from an Apple Watch if you don't have (or plan to get) an iPhone?
It's hard to say, considering we don't have access to any material on it other than the website and the keynote. It's likely you've already reviewed all public material on the device.

To quote the bottom of the WATCH page, emphasis added:


  Features are subject to change and may not be available in all regions or all languages. Requires iPhone 5 or later.


I bet the watch can't even be activated without an iPhone. Due to power requirements of Wifi, I doubt the device will carry an 802.11 radio, preferring to get it's data through the phone.

(This answer is entirely speculation and may be invalidated as we learn more about the device.)