Some thing that could be improved on
The email to say that the device was on its way could have the company name in it or something to indicate where the email is from. (note when I queried the email, stevies response was prompt)

kiwijunglist:
+1 - I thought the email was spam.  was complicated to work out what to do with the tracking number, as when you visit the castle website there are 5 different types of tracking numbers you can enter, and it wasn't clear from the email which to use.

eXDee: Arrived today.

Was disappointed there seems to be no Play Store link for the app? Went to the URL on the little card it came with for the APK and it went into an infinite loop, it opened about a hundred tabs in chrome before i stopped it. Not sure why.
For anyone who can't read the tiny font on the card, its:
http://axgio.com/downloads/gkey/glick.apk

I downloaded the apk on my PC and used pushbullet to push it to my phone.

For anyone wanting to know its permissions, it has a fair few as i imagine its a multi purpose task launcher.
Four images:
https://imgur.com/EBa2isZ,0xSz66z,wMNEIL1,fOXLwGj

At this point I discovered the more popular "pressy" button requires you to have an activation code to use their app. Lame!

edit 9001: User error! Originally i said it didnt work with my Nexus 5. It does work, i simply wasn't clicking it hard enough. My button would make a click sound, but you have to push it firmly down for it to register.
This makes double tapping or especially triple tapping much harder, you have to do it firmly every time on my unit.

The Klick app tells me long press isn't supported. It does however pick up the single and double clicks.



edit: The item itself looks nearly identical to this. The description also references the same "Klick" app, as well as another app.
http://www.dx.com/p/3-5mm-plug-shortcut-key-for-android-phone-silver-315648

The Klick app is here on the store:
https://play.google.com/store/apps/details?id=com.helloklick.android
When you install it, it has an english version if your phone is in english and appears to be the same as the apk above and the version number is the same.

I got the same result above with my phone however. Mentioned in that DX listing is anothe app called '360'. I installed this from the play store but it does not seem to have an english version, only the originating language. 

edit2: Xiaomi MiKey app is an alternative app. I can confirm it works with this:
http://xiaomi.eu/community/threads/mikey-app-1-7-1.24220/



If you want to test it and eliminate the app being a problem, simply plug it in and don't install anything!
It acts as a play/pause button in your music app, same as the button on a headphone jack would. And if you long press it, it'll open Google Now:



Further confirmed by it working in this app for inline audio control buttons:

freitasm: Interesting, it didn't work my HTC One M7 (stock) at all.

Are you using the stock ROM? Standard launcher?