I paid for prep and it was absolutely worth it for me. But I hadn't really done an interview for a big company in well over a decade. My interview at Sun Country was a much more casual affair and not one on which to base the experience with a major airline. No negatives towards Sun Country, but they're different and much smaller.

Anyway, the prep I did was not cheap but was 100% worth it because they helped me formulate my thoughts into a more concise presentation. It gave me much more confidence that I was ready and they really double checked all the paperwork. I felt that I was as ready as it was possible to be when I walked in that morning. And I had the best interview I've ever had, in large part because the preparation put me at ease.

This was my experience and everyone has to do their own thing. I know many people get hired with no preparation from a paid service. It really depends on the individual. If I were going to interview at another major carrier (I'm not) I would definitely pay for prep from somebody who knows the process of that particular carrier.