I went to the London event today with a friend - managed to play 5-10 minutes each of Snipperclips, Zelda, Mario Kart, Splatoon 2, ARMS (surprised how fun this is), three 1-2 Switch minigames (Milking, Guess the balls and the quick draw one).
I came away feeling positive from all of them really and definitely have a much better impression of the Switch now. The Pro controller feels good but I don't believe its justifying the higher asking cost than the Dualshock / Xbox controller whilst the joycons attached to the grip do feel a little narrow. However these are pretty small negatives given how nice the tablet is particularly the screen and how fun the games are. It was good trying the different control methods as well as how easy it is to dock and undock the tablet.
It was a really well organied event by Nintendo being able to get some bitesize experience ahead of next weeks launch. I'm definitely going to get one for Mario in the holiday but if there's a good deal sooner I'll pick one up sooner.