Here's what I've been playing today:
Snipperclips: A wonderful co-op game, where you and your partner play sentient (very expressive) bits of paper that cut bits out of each other to make shapes and complete puzzles. An absolute joy to play with nicely involved puzzles. It is pretty much limited to local co-op though, so its worth ultimately comes down to how much of that you're going to get to do.
1,2 Switch: Some fun minigames. The quick draw game is a lot of fun, but I can't see it having much longevity, even as a party game. After three quick duels, I was pretty much fulfilled with it. The "how many balls are in the box" game is technically impressive, but even the staffer demonstrating it couldn't muster up enthusiasm for it.
Mario Kart 8 Deluxe: It's just Mario Kart 8, but with an extra item slot. Seriously, felt 98% identical to the Wii U version.
ARMS: Had a quick go on this. There's a decent amount of depth to it - half a dozen characters, multiple weapon options for each, but ultimately it comes down to the motion controls again and... yeah, it just felt too similar to Wii Sports' boxing for my liking. The motion controls are more nuanced than that (you can throw a curving punch with a hook for instance) but it ultimately felt like a lot of flailing, possibly because it was my first use of the JoyCons.
Splatoon 2: Again, pretty similar to the Wii U version, albeit with a twin blasters option now. I struggled to get on with this, as you have to aim with both the right analogue stick (lateral movement) and the tilt sensors in the controller (looking up and down) which is just confusing as hell. Didn't appear to be a non-motion control option. Beyond that, it's pretty much the same game as before.
Zelda: Breath of the Wild - Beautiful. So fluid and gorgeous. The controls felt a little odd, again partly due to unfamiliarity with the controllers, but the addition of a jump button is long overdue for Zelda. Switching weapons is a bit of a pain though, as you have to hold right on the D-Pad and then select with the right analogue stick, rather than a quick cycling option. The demo was the E3 demo from last year and, as I picked up from the previous attendee's session, gave me an opportunity to wander around a bit, get some items and fight a group of Bokoblins, all of which was a lot of fun.
Overall impressions of the Switch: There are so many controller and play options and they were all evident at the event. Splatoon was running 8 player local multiplayer on TVs and using joycon grips, Mario Kart was doing 8 player local multi using tablet mode, Snipperclips was in tabletop mode, Zelda was hooked up to TVs with a pro controller, but we were able to switch to the handheld mode, which really is as seemless as it looks. The controllers are all solidly built, but don't have the immediacy of say the GameCube controller (which instantly felt right) but that might be as much to do with the environment. The individual JoyCons felt a little awkward for 1, 2, Switch and Arms (as much because of not knowing where all the buttons were) but were great for Snipperclips. The JoyCon Grip is good and the Pro Controller is also solid, though not as nice as the Wii U Pro Controller, imo.
I wouldn't say going to this preview event changed any of my opinions about anything for the Switch. It's a nice bit of kit and the games are generally good, but it's still too expensive and doesn't demand a pre-order, especially if you've got a Wii U.