Splitting this up comment into my common use cases.

IoT:

All my lights are Phillips Hue. Outside of the Echo I have HomeKit set up to take care of automatically turning the lights off when I leave my apartment's geofencing zone. When I arrive home from work, I really enjoy opening my door and saying "Alexa, all lights on" followed with ripping my headphones out of my phone and saying "Alexa, Play Spotify" to have her transfer the Spotify session over (more on this in the next section).

It sounds really silly when I look at it now, but honestly when I'm traveling and I stay at a hotel or something, I definitely miss not having to think about turning all the lights off. It's an extra task that's very lightweight, but it feels good to not have to think about. Before I had the HomeKit compatible Phillips Hue bridge I said "Alexa, all lights off" when leaving my apartment every day. That too is a lot easier to do than recognizing that one of my lights is on and needing to go to the room to turn the switch manually.

Music:

I have a real sound system in my apartment but it's so much easier to just have the Echo play music, and it sounds pretty good.

If you use Spotify, you can use the Spotify app to directly stream to your Echo. You can also just be listening to an album or playlist on your phone or Spotify on your computer, say "Alexa play Spotify", and it'll pick up on the same part of the same song with the same queued up next songs. The Amazon Echo competitors I've tried don't do this, they start a new Spotify session and play whatever they feel like (e.g. Google Home defaults to the first playlist in your playlists list.)

Cooking:

I rely on the Echo while cooking a lot. Everything from "Alexa set a timer for 20 minutes" while roasting some potatoes to having it do the calculations for reducing the recipe's ingredients by 1/3 since I'm cooking for one and it's a recipe for three. I like cooking with the Echo a lot more than with the competitors because on the Echo you have a giant blue ring that shows you if it heard you or not. I burnt my sweet potatoes the first time I tried cooking with the Google Home because it has a "face" that faces a specific direction and my oven is outside of that field of view, so I didn't know that it didn't hear my "Okay Google" and set no timer.

API wise:

The "Skills" themselves are really easy to make, but I wish the communication for using your custom skills wasn't so clunky. The only thing that I've actually made with their real API is a simple interface for Halo achievements.

My xbox is in my living room near my Alexa and my friends and I routinely would see an achievement pop up like "Top Gun" and want to know what it meant, but since we're in the middle of the game we can't check now and we'd forget after the game.

In less than 30 minutes I was able to write some JavaScript in the Chrome console to scrape the list of achievement name and descriptions for the Halo 5 website, then make a Skill out of those. Since the list of medals is pretty static, this is a set-once and forget operation.

Now when I'm playing on my Xbox, if an interesting achievement pops up I can say "Hey Alexa, Ask Halo what's Killtacular?" and it responds with the description. The "Alexa, Ask Halo" is the clunkiness I mean. I wish I could just say "In Halo, whats Killtacular?"

For what it's worth, the Skills editor has a really decent flow. Here's what the Skill I'm describing looks like in the interface: http://i.imgur.com/8VPbt0v.png