I really enjoyed the system, and that’s saying quite a bit considering only one of the players had more than a rudimentary command over the rules, and our GM wasn’t much more informed. Truthfully, you just don’t have to be that knowledgeable because it isn’t that complex (for the most part), and I can say that even though I played a bard, which is technically the most complicated character class, and the game was an 8th level one-shot. There were definitely a few things any given player would forget from time to time, but they mostly centered on damage expressions. It seems like the players were constantly apologizing for the amount of damage they were doing because, at 8th level, it’s a ton.
I also love the flavor of the icon structure, which we all used, but enough has been written on that already. You are free to form your own opinions (or assumptions for those who haven’t played yet).
My verdict is that it’s a system that does exactly what it intends to do: Take the good parts from D&D 3.5 and 4e, and get rid of the bad parts. (Full disclosure: I’m a 4e player.) Despite our confusion with the system and the fact we were playing 8th level characters built the week before we played, the combats were fast and furious. The escalation die wasn’t much of a factor for any of us that I could tell, so once we start using that to our advantage, it’ll likely go even faster.
If you get a chance to play 13th Age, play it.