how are partner interviews?

Is this for IB or PE? It doesn't really matter, I guess. The partners don't care about technicals. They assume the other interviews have sorted that out nor do they remember a lot of the basics. Maybe (?!) 1/10 MDs could walk a random change through the financial statements. 

That isn't their job and as you age...a lot of that doesn't matter. Their job is to have relationships within the industry and be able to make a call to the right person at the acquirer. Building those relationships requires a ton of work. All they need to know are the current comps and how to position the company to do better than that for the right buyer. For PE it's a simple assessment of does this person get it. Partner interviews are the best because you might actually learn something from someone who is in their seat for a reason.

Simply put, the interviews at a partner level are easy. They just want to like you and see how well you'd fit in. Ask smart questions and let them talk. They also want to like you so they don't have to sit through 20 more interviews.

Funny partner story from my past...I was interviewing at Polaris as a 3rd year at a BB, which was a much bigger VC/GE name back in the day. I was being pushed hard by a partner who also graduated from my semi-target, for an early-stage VC role (not their Summit/TA style growth equity stuff). I had an amazing conversation with Bob Metcalfe about the foundation of the Internet (Metcalfe's law, ethernet inventor, etc.). He told me that he wanted me to work for him directly...sorry, I'm a geek, but I relish that conversation to this day. He was very cool. 2 hours later I met with another founder of the fund, who clearly didn't like me from the minute I walked into his office. He looked at me, told me to sit down and then proceeded to ask random, easy questions without paying attention to my answers as he was emailing the entire time. 

My partner level contact was baffled as he shuffled me through 8 rounds of interviews, and he said, I don't know what to say but the one guy really didn't like you, everyone else thought you were great (like 20 people). Maybe his son-in-law was the other candidate? No idea. Regardless, meeting Bob Metcalfe was more than worth it. Obviously, I didn't get the job. They had a rule at the time that the 3 founding partners had to sign off on any VC hires so I went out with the trash that day.