Great for some frantic on-the-go action, but definitely toned down a bit due to DS limitations.
In Geometry Wars you get a simple ship that can shoot bullets out in all directions. You start off with a gun that shoots a two bullet stream and eventually upgrade to 2 higher level guns. One shoots a fast 3 bullet stream, one shoots a slower 5 bullet stream and they randomly swap between the two. You are then bombarded with randomly appearing enemies all around you that die in one hit, but so many appear that it quickly becmoes a frantic fight for your life.
In Retro Evolved (the Xbox Live Arcade version) you simply had a square grid area that was slightly bigger than the screen and the difficulty ramped up fairly quickly on you as you attempted at a high score. In Galaxies the formula has changed up quite a bit. There are now several different room shapes, more enemies and you even get a secondary ship with you in all the levels.
You first pick a galaxy to play in then you can pick from several different planets within each galaxy to play in. There are roughly a dozen galaxies and 4 to 7ish planets in each galaxy. Each one of these boasts a different challenge and none are to similar to be honest.
Some have you in a square room just like retro evolved, but with the newer enemies and secondary ships you're strategies will have to change a bit. Some however will put you in diamond shaped rooms, real funky shaped rooms or rooms with barriers (some even have moving barriers) in them. This really messes with your mind and causes you to have to switch up your playing style based on where you are.
As I mentioned, there are now secondary ships for you to take with you into battle. At the beginning of the game you have one that simply attacks with you. As you play levels and collect geoms (dropped by enemies you kill) you'll actually level up this ship. I believe they all max out at level 9. Geoms can also allow you to by more of these ships. There is a defense one, a turret one, a bait one, one that helps collect geoms and more. This gives some extra strategy to this crazy frantic game and also gives some replay in leveling these guys up.
The game is really great, a lot of fun, but it has some flaws. If you've played Geometry Wars you know that the screen can get quite chaotic and filled with hundreds of enemies chasing you down. Well, the DS doesn't always like this and has major slowdow when you get to many enemies on screen. If you are in the running for a gold medal on practically any level you're going to be experiencing this slowdown.
The controls can also be quite difficult to get used to. I tried out all the control schemes from using the D-pad for movement and the face buttons for shooting, the bottom touchscreen for aiming while the playing field is on the top and the playing field on the bottom while I touch in the direction I want to shoot. I found the middle one the best after I finally got used to it. But the D-Pad and holding the stylus through fierce battles can become quite painful on the hands and doesn't always feel like you have total control, not like the dual joysticks of the 360.
The other thing is that since the DS has to run sprite based graphics, you don't get those fancy grid effects in the background. Until you've played without them you don't realize how much they really do help you play. You can tell where black holes are easier, where your shooting, etc. Once again something that can be overcome with time, but it's sad to not see there.
All in all this is a quality game. For some Geometry Wars action on the go you can't really go wrong here. There is a lot to play, a lot to experience and quite a bit of replay value so don't miss it.