Only a couple 3rd party apps have been usable for me.
Uber is great, Clear is great for shopping lists, and Dark Sky is better than the default Weather app.
Some apps are beyond useless, like Shazam for example.
Took me a while to realise Shazam isn't actually doing its listening on the watch, it listens to music using the iphone!
My phone is usually in my pocket, so it's not exactly a great place to listen for music, which means I have to get my phone out of my pocket anyway so it can listen to the music. Completely and utterly pointless.