With KL/AF you can just scan the BP as it recognizes your status.
With Alitalia you are given a paper pass if there is a problem with the system recognizing your FB status, which has happened to me today.
When the new fast track channel works well at BCN, it works really well as no regular passengers can be "merged" into the line, as happens in some airports. BCN is enjoying a record-breaking year with terminal 1 fast heading towards capacity, especially during the peak summer period, which makes Fast Track all the more valuable.