Mac ID works GREAT on my watch...
Just downloaded Mac ID for iOS onto my iPhone 6. This app allows you to lock or unlock your Mac with your paired Apple Watch.
I have two Mac Minis, a mid-2011 (for general web use) and a late 2012 model (used with Parallels and Windows 7 for running 3D CAD). Both machines are running the latest version of Yosemite.
First, download the iOS app to your iPhone. On your Mac, Google Mac ID, go to their website, and follow the instructions to enter the unlock passcode for your Mac (NOT your Apple ID) and the encrypted file is saved.
Then, in the Apple Watch app on your phone, enable the Mac ID app in Glances for the watch. I then re-ordered the glances screen to show the Mac ID as the first screen (you may desire a different order).
The UI on the watch is good. It shows the current lock/unlock status, and a simple press toggles the lock state. If you have multiple Macs, you can scroll up to display the icon associated with that particular Mac, and then toggle the lock. Additionally, the Wi-Fi network is listed, the battery state of the paired iPhone is shown, and you can also control your iTunes music play status within the glance screen (although I have yet to use this feature).
So far, I'm really pleased! Two thumbs up, and highly recommended.