It's even harder than the original, but the other improvements push it above its predecessor and make it worth a try.
Negative: - only 8 levels in the main game - still REALLY hard, even on easy -
Nanostray was a good game that was fun but way too hard all the way throughout, even on easy. Now, Nanostray is back and harder than ever, but the sequel is better than the first in many ways.
The main part of Nanostray 2 is the Adventure mode. This only has eight levels, but it's so hard that you'll never know what the fifth level is like, even if you choose 'easy' (which, for the record, is NOT easy at all). There is even a story this time around. There's some virus that you need to stop and all that stuff. It's not necessarily an interesting story at all but it makes you feel like you have more purpose in playing than just going around shooting stuff like in the original. The levels are a lot longer this time around, which can be a good thing to give it more value but again it makes the game more difficult.
Aside from the main Adventure, there are a bunch of challenges that are actually really hard like the rest of the game, but they get pretty addictive. I think there are 24 of these challenges, but like in the adventure, you'll probably never go beyond class A into the ninth challenge and beyond. These challenges vary from getting a certain score within a time limit to surviving a short period of time.
A good improvement on the series seen here is an entirely different view in half of the levels. In the first game, all the levels were from a top-down view, but this time there are several sidescrolling levels as well. This varies the game a bit, which is pretty nice, and the sidescrolling levels can get pretty cool with twisting cameras and a different feel.
The weapons system has been greatly improved from the original game. Rather than having to touch a spot on the bottom screen to change your weapon mid-level, you select your secondary weapon to use in the level before you start playing and there is only one primary weapon, but you will quickly collect two power ups that make you shoot out three beams at a time, and you can shoot in several directions around you by pressing R. This is a nice gameplay mechanic and nicely makes up for only having one primary weapon.
The graphics and sound in Nanostray 2 are good as well. The visuals are nice for the DS and the enemies are sometimes cooler and less generic than in the first game. The sound is cool and the effects for shooting and stuff are fun to listen to.
So all in all, Nanostray 2 is a great game. If you played the original but wanted something better, this is pretty much what you'll want to pick up. If you want to try this series for the first time, it's also good for that too. Even though difficulty of it will likely kill some of the value for most players, Nanostray 2 is still a great, fun game likely worth its cheap pricetag.