I also had such a lunch on an on-site interview day and must say that I really liked it. Not only can the company assess your fit but you can also assess whether you like the company and your future colleagues.
Indeed, I've gone through a similar process and it was really nice for both sides.
Although I believe that this should be done carefully, because when people get close and personal, they tend to "judge" others based on the personality, which is important, but not really what they are doing, instead they should focus on the cultural fit having in account the candidate's personality and other factors.