I use them, when I am connecting in FRA (to be honest, this is not very often). It gets you in peak time in less than a minute through border control. But I have had some problems in the past, too.

The scanner for your passport needs the whole page with the maschin readable identiy card. The cover of the passport is bigger than the card with you personal information. If the scanner rejects my passport, I try again and be sure, that the page with the card is on the scanner and not the cover.