I've done 12s. Some were awful. Like... 9 in a row, then 2 days off, then 9 more... Wasn't a fan of that one.
But two 12s, then two 12s (night) then four off was nice.
We have a department that struggled to keep people and they had was working 12s 7 days a week for awhile. They would shut down occasionally for the weekend. I personally like 12s as long as i don't have to work more than 3 in a row.