I did a 10 session package (got it on sale) that focused on (distributed) system design and ML system design. I felt confident in the coding piece, but I didn't have the opportunity to work on large scale systems in my previous role. I believed that I could obtain a sufficient level of knowledge by self-study and practice, but I also have a toddler that takes up a lot of time and energy, so I wanted to be more efficient. I think it was valuable to absorb knowledge from a mentor that has real experience working in my target role. It turned out that was very successful in my real interviews. I think that I would have still done OK had I not done the interviewing.io, but, by my estimate, the return on investment was high, since I received multiple offers and negotiated up.

e: I wanted to add that my mentor and I decided that we would do 4 pure teaching sessions, and then the remainder would be partial mock interviews that would transition over sessions from them doing most of the talking to me doing most of the talking. I found this format to be good for building my own confidence.