I went in for a three person interview but instead of a panel, it was three separate interviews- meaning half of my day.
The interviewers were all very nice and honed in on different skill sets which I appreciated.
Their record keeping/follow-up however, was awful. I sent thank you emails to all three interviewers the following morning and one of the three replied that it was nice meeting me. Other than that, I heard nothing- not even "sorry, we're going with another candidate." I did reach out once more to the hiring manager two weeks later and still heard nothing.
About three weeks later, the one person who had originally responded to my "thank you email" sent me the generic interest email asking to schedule an intro call to discuss my work history for the position. I politely replied that I'd already had a call with [hiring manager] and had also met her (as well as the hiring manager and an additional manager) and gave the date of that previous interview. There was no reply and, seven months later, they're still continually reposting the job on all of the job search sites.