So I ran yesterday with the watch and used Runkeeper. It was ok, but I'm not crazy about the Runkeeper interface of the watch which is not customizable at all.
Today I ran a 5 mile race with the Watch and used Endomondo. I'm not using the stock Apple Exercise app on the watch since it doesn't provide any real details on the run or mapping tools with the GPS.
Endomondo is MUCH better than Runkeeper on the watch. I tweaked the interface so it had the fields I care most about up front (pace, average pace and distance), with secondary fields a swipe away.
There are some definite limitations at this point though, and I think it's more with the Watch than Endomondo, and hopefully these will get fixed in an update. First, the watch had my distance at 4.8 miles after the race, and not 5, which it definitely should have been. I could ignore a few hundreds of a mile either way, but 2 tenths is way too much of a variance. Not sure if this is an issue with the Endomondo app or the GPS on the iPhone, but I always used to find the iPhone's GPS more accurate when I used to run with it.
Also, when I want to look at my watch to check my pace or distance, especially in a race like today. I have to wait for the screen to wake up, which is pretty fast and no big deal, but then I need to wait for the app to show my current stats. when it woke up, it was still showing the stats from the last time I checked it and it had to refresh, which took a couple of seconds each time. This is a big deal. To have to keep your wrist up to wait for the screen to refresh will cause you to slow down and take your eyes off the road. Not good for any runner if you're trying to maintain a pace.
Minor issues were the same as Runkeeper. Neither app is integrated with the Apple exercise app, so I don't get any taps on my wrist when I hit each mile. And the lack of integration with the heart sensor is also a major missed opportunity but I'd like to this that will get resolved with an update.
Now I'm coming off using a Microsoft Band since October, and a Pebble before that (yes, I love my wearables). This is probably the one area the Band beats the watch, at least for now. Yes, the Band has a very accurate GPS built in, but I had the ability to keep the screen on throughout a run, which means it doesn't have to refresh the data. Even if I turned the screen off and then on again when I wanted to check my stats, the refresh was very quick. It would be better if there was a setting to keep the screen on without timing out. Yes it would eat the battery but I think most runners would want that. And speaking about the battery, I was pleasantly surprised. It was down to about 95% when I started the race, and after a little over 30 minutes with endomondo running and lots of data going between the watch & phone, it only dropped to 88% by the time I was done. I expected a bigger hit to the battery with a run like that.
I'm hoping these issues get fixed with updates, because I really do like the Watch so far. I would also think Apple would update their exercise app to make it more full-featured and GPS-enabled.