Went through Arrecife last week and had to queue to get registered on arrival (30 mins, not too bad). However, we had already registered in Alicante last month. This didn't cut any ice with the staff, everyone had to try to register again. It scanned my passport and sent me on my way without taking another photo or fingerprints. I think they (rightly) assume that if they give people a choice, and people see a way to skip the queue, everyone will claim to have registered regardless. Maybe a big official "I have registered" sticker on your passport would help.
No issues on departure. Everyone was told to use the machines in the lounge before going to the gate (which we did, quick scan, no photo or fingerprints again). They did start boarding 75 mins before departure, so must have been expecting problems. I did like that the machines are at the gate so you don't need to go through into special non-Schengen "holding pen" like at some airports.