I really like what I've played so far. The controls take a bit of getting used to [and tweaking them a bit could have made them better] and it's a bit too eager to give up it's secrets, but it's definitely Metroid. Feels like it's been forever since we've had a proper 2D Metroid, Zero Mission is over 13 years old now [no, Other M doesn't count and also it's bad]. The counter attack is okay, I dunno if it really adds that much but it works well enough once you figure out the timing and it gives the developer's an excuse to use enemies that have slightly different attacks patterns from normal. I wouldn't miss it if it wasn't there, but it's inclusion also doesn't bother me. Graphics are pretty nice for the 3DS, meaning they're dated by current standards, but solid art design helps to patch over some of the deficiencies.
Also, remixed Metroid 2 OST and reworked Prime series tracks are pretty nice: