The queue was fairly short when I got there so I didn't have to wait long to play it (about 15-25 minutes), but I saw it get much bigger at various points throughout the day.
Before I say anything, I would like to note that I haven't played through Shattered Crystal. However, I do own the e-shop demo for it and have some spare plays on it, so I made a point of playing through the first level of that before the event (I did it on my plane ride over to London on Friday morning). My general opinion on that side-scrolling gameplay from the demo is that the game feels fine mechanically, but it has a few things I didn't like and the levels do drag on since you don't always know what is the "right way" to go. Often, you'll go somewhere, thinking it's a higher, faster route like in a Classic Sonic game, only to find it leads to a collectible, and then you have to loop back around to where you were. It's not the most satisfying way to design a Sonic level.
Fire & Ice is definitely a big improvement on the formula, in my opinion. For one, I played through the level in Fire & Ice's demo with only Sonic, while I would often have to switch to someone else in Shattered Crystal. I can't even recall if you could switch characters in the demo I played since I never had to attempt to. I also completed the level in about 4-5 minutes, and that was with me doing some exploring and testing out the controls at the beginning of the level (I completed Studioplolis Zone in Mania in a similar time frame). The levels are laid out more like a traditional 2D Sonic level, it's actually comparable to the Classics in that sense, with higher routes generally being faster but also having more collectibles. The only notable difference is that especially fast sections with loops and the like are still relegated to being mostly automated and you don't really get to use slopes to your advantage in any way. However, I don't consider that inherently bad, since this is not supposed to be a Classic Sonic style game (that's what Mania is for). It could also just be that this particular level was a simplified one.
I did find secrets and such hidden in small corners, but when I did, they were literally just in a single room with not much in the way. It was like finding a Special Ring in S3&K. I found a locked door, but couldn't get in. Either that was a demo restriction, or you have to find a key somewhere in the level, or maybe even in a different level.
Also, in Shattered Crystal, one thing that bothers me is when you do Sonic's Air Dash move down and into the ground, he gets stuck in a recoil animation for a couple of seconds, which breaks the flow. In Fire & Ice, he will instead bounce off the ground as if he hit something with the Homing Attack, so he can still move around, or even perform an actual Homing Attack. Or you can Air Dash again. This makes the move a lot more satisfying IMO.
I couldn't listen to the music though, as there were no headphones and well, there was so much else going on.
Overall, I enjoyed what I played (just the one level, so I can't comment on the tube levels and such). I might consider getting the game if I have the money.