I've only met artists by hanging around the venue before or after the show, not from an official meet and greet. But anyway, everyone I met was super nice: Emeli Sande, Ed Sheeran, Betty Who, Lykke Li, etc.
Oh, when I met Fifth Harmony that was an official M&G type situation (where you just waited in line, then got to meet them/take a pic) and they were nice and all, but it was pretty rushed so I only talked to Queen C really.
I had an extremely short M&G with Katy when Killer Queen was released in my country. She was extremely sweet, but I don't really remember any details because I was so excited