Does the built-in Google app data backup work? This question is a long time coming I guess. I have been seeing claims in Android since the dark ages that our data is being backed-up to the cloud, and recently "Sync App Data" showed up as well in the account settings. However when I have to restore, all I get is the apps I had, and a very small restore.. perhaps wifi passwords and desktop wall paper from what I see. So my question is this..

Does the in built-in backup service work?

I see from code documentation, that the apps need to manage it manually. I guess then it is possible that some apps do not support this?
Is this backup service actually supposed to backup app data or only just system data?
Is installing helium or similar 3rd party services the only way to go?

Second minor question. Am I right in understanding that Helium and its adb backup bretheren rely on the opt in android service, and that Titanium backup and their ilk just copy all possible files of the app and thus do not require the opt in service?

p.s. Am sorry if there is a good post about these details, much of what I see is very cynical posts which plug one of the aforementioned 3rd party software.
Does the in built-in backup service work? Yes.

But maybe not how you expect/want. As you mentioned apps need to 'opt in' to backup data and most apps don't actually use this. I'm only aware of a couple that do. As far as i know, the backed up data is only actually restore to a device when the device is initially set up, not when you install an app from the Play store that you've previously used (i could be wrong on this though). Google recently launched a way for games to sync progress between devices which more apps seem to be using and are publicising this more.

Helium and adb backup work differently in that they backup the data stored on the device and don't require opt in. Helium uses the adb backup method so essentially they work the same way (although it doesn't require a pc to perform the backup).

Titanium Backup backs up the apk and the appropriate 'data' folder (NOT the folder on your sd card by default) and again this does not require any opt in from the app developer.

Personally i use Titanium Backup as it was the only option when i first rooted my first phone. I've not used Helium so can't say if it's better/worse. When i have used adb backup (not via Helium) it took a long time to backup/restore but it does work quite well.