So I've been using the workout app for my runs since getting the Watch the Tuesday after launch. I also have a Garmin 620. I always run with my phone.
If I only wanted a watch for running I would definitely stick with Garmin at this point, and I'd wait until the 225 or inevitable 625 is released with integrated HRM. The display is better, the accuracy is better – it's just made perfectly for running.
But I've been sticking with the Apple Watch because I like the other metrics and functionality – daily calorie burn, steps, call and message notification while running. But I've noticed my distance has been erratic this week, and I've run the exact same 6 mile route for 4 days. I'm also finding pausing and resuming at street lights to be less than ideal, especially with sweaty hands. And I can make visibility getter by turning up brightness and changing the font to bold, but the Garmin display is better for running.
What I'm finding most frustrating is that for the three 17-mile bike rides I've taken since using the Watch, the mileage for those has been exactly the same for each ride. And some of the runs I've taken with the Watch seemed spot on in the first week or two, but now they're fairly erratic. I'm starting to think since I've put so much mileage in the Workout app that it's not using the Watch for GPS anymore, and just basing distance on steps/stride/etc. I have no way to know this for sure, but that's my guess.
I'm going to try some of the third party apps in the next week or so and see if those fare any better. I'm not thrilled losing the HRM data, but the mileage inconsistency has been frustrating this week.