This isn't necessarily ESPN's fault. Its app is at the mercy of whatever network you happen to be connected to, and if you're out and about, that can change as rapidly as the momentum in a Stanley Cup playoff game.
I had my best success with the app when I stayed in one place with a strong Wi-Fi or 3G signal--I was able to watch a France-Serbia soccer friendly or the latest episode of SportsCenter without any show-stopping problems. Unfortunately, that doesn't really mirror the most likely use case for WatchESPN: You're likely to use the app when you're on the go. I tried watching a baseball game on my evening commute ride home on public transit, for example, and would lose the feed anytime my ride hit an area where AT&T's network coverage was less than optimal. Again, ESPN can't do much about that, but when all I want to do is check in on a baseball game and I can't, I don't really care whether the app or my wireless carrier is at fault.
To ESPN's credit, it does a good job with the things that are in its control. Besides the smooth login process, WatchESPN does make it very easy to find programming to watch, whether it's live events or replays of already-completed games. The app also offers multiple ways to search for upcoming events by sport or channel. That said, the app only offers a list of upcoming events--it would be nice if I could set a reminder that the app will be streaming next week's Poland-Greece European Championships opener rather than having to rely on my dodgy memory.
An app like WatchESPN exists so that rabid sports fans like myself can take comfort in the knowledge that a must-see game is only just a few taps away. For that app to be a success, I need to know that if I happen to be away from my TV when some athlete somewhere is doing something amazing, all I have to do is take out my iPhone, fire up WatchESPN, and enjoy the sweet relief of live video. The app can't do that dependably--not at this point, at any rate. It's worth having on your iOS device as an option for viewing live sports, but you need to be ready to live with WatchESPN's current limitations.
Philip Michaels is Macworld.com's editor. He enjoys watching sports because of his dull interior life.