I have been playing more of this over the last week or so, and I'm on Floor the 7th. I feel like I'm prepared to give my final opinion of the game.
First, the concept. It's one part RPG, one part DDR. The RPG aspects are limited to your stats and equipment (meaning there is no exploration or questing involved), but those things aren't what the game is about so they aren't missed. The DDR aspect is executed brilliantly as well, requiring attention to not just one but up to 3 different sets of falling arrows.
There are so many ways that Sequence could fail, and it deftly avoids all of them:
The core gameplay mechanic could be executed poorly. Changing fields is immediate and easy to do. You really only have to pay attention to 1 pane, and the others don't matter unless you're either using a spell or trying to recover mana. The step recognition is solid and pretty forgiving. Also, the steps are randomized so you don't just end up memorizing an optimal pattern, but they still match the beats in the music. That by itself is a huge accomplishment.
The music could stink. Any rhythm game lives and dies by the quality of its music. The soundtrack is slick, catchy, and doesn't wear out its welcome with multiple plays. My only complaint is that I want to hear more tracks! But that's what DLC and sequel(s) are for, I guess.
The game could rely on the RNG too much. Unlike the recent-ish Wizardry game on the PS3, this isn't you vs. the RNG. The RNG does play a factor in synthing items, drops, and the battles, but ultimately your success is determined by your skills on the keyboard/guitar/dance mat.
Ultimately, the game is a success on every level. The gameplay is frantic, the music is excellent, the writing is awesome, and even the voice acting is far above what you'd expect from an indie title. At $4.99, it's an incredible deal.