We paid for priority plus for our recent Mexico cruise on the Joy as we were unable to upgrade so added on a few perks (premium beverage package as well).
After doing this once, we wouldn't do it again. We got to the terminal early (10am) - there's no separate line for the security screening - we walked right up to the regular check in that we were directed to (after saying we had priority boarding). I couldn't see any specific check-in area for priority boarding - the security staff told us to go down "to the end" towards the concierge checkin but the regular checkin had no line so we ended up there. The check in staff identified that we had the priority package and told us to wait with the rest of the group and that priority would be called up to board early-on.
I asked about a separate waiting area as I'd seen pictures on a previous board about that - there doesn't seem to be anything in San Pedro - the concierge area was packed, I think that's where they would have sent us if there was room (it was nothing special at all just a cordoned off area with the same seats) - the lady I asked told me to go to guest services once I was on if there wasn't a designated area but I didn't bother.
We were called to board after concierge, before platinum/platinum plus... then groups one, two etc. In the end groups one and two were pretty much all in the same line to go up escalators and I suspect we would have been in an early group given our early arrival anyways. Maybe would have been worth it if we'd arrived later as the lineup seemed to stay quite long throughout the day for at least a couple of hours after we boarded.
Canapes on 2nd day were nothing special at all. We didn't have to get tickets for the cabo tender and just went when we were ready, so that was nice. I was most worried about disembarking but we carried off our own luggage anyways so were off super early regardless of priority boarding.
We didn't use the spa credit - and as others have said it's only for regular price and only on port days (and there were lots of specials that I think might have been almost as good a value anyways).
Live and learn 🙂
Hope that helps for future folks - was hard to find much info on this as I think it's pretty new.