So I tried mine for the first time 2 days ago (SFO-LHR). I checked in via the app, and added the TAG details - all worked OK. I even had a digital receipt within the app.

On arriving at SFO I actually had 2 bags to check in - but the app only supports 1 (for now). Mrs Avon75 used my spare for her suitcase.

At the desk the attendant said there was a TAG assigned to my booking, and was a bit confused because I had needed some kind of paper thing (she kept gesturing towards the baggage drop off section). Anyway - we both checked in fine and received our luggage as usual at T5 (i.e. significantly after everyone else despite being 'priority'). No damage to either (which was applied to the middle handle on both based on my strategic observations of how luggage ends up falling off the conveyer belt).

Interestingly, both suitcases had a paper tag added, in addition to the digital TAG.

Based on my first TAG experience, I don't know why I bothered.