This is a good game, 8/10. I'm going to start this review with my good experience with it, then the bad one.
Gundeadligne is a japanese horizontal shmup, so you can expect it to be fast, colorful and a good description of "bullet hell", since you can find the screen to be covered with bullets often. What made me like this game is that I always am in a rush feeling playing it; the nicely arranged soundtrack gets you pumped up for action. Also, when you play with the main character Eryth, you have to repeatedly press the shoot button to fire both of her revolvers at the same time, doing more damage, and making the gameplay feel more intense.
The pixelated graphics looks awesome for me, but some of my friends disagree with that. The enemies in this game are generally half-naked girls, but in no way that is some kind of sexual opportunism, as it is not the focus of the game.
The story is not so great if you care to that (I don't, really). At the start of every stage some text explains what is happening.
The multiplayer mode is a lot fun to play, up to 2 players, local or online. The online multiplayer won't work your friend is from the other side of the world, that makes every action you make have a 1 to 2 seconds of delay.
I would not recommend this game to be someone's first shmup, and one thing to justify that is that you use four to five keys often to play this game, and I would feel confused if it were my first time facing it. Also, the game gets harder if you are being good at it. The difficulty of the game might not be appropriate for shmup beginners.
Now with my bad experience with the game.
As I mentioned before, if you're doing good in the game the Phase Level increases, making the game harder. If you play it in the easiest difficulty, that is novice, the Phase Level starts in 1 and won't go past 10, that is suitable for the difficulty. However, if you play it in Standard, the second difficulty, the Phase Level starts in 6 and goes up to 21, that is the maximum. The thing is that the Phase Level increases too fast for me, because I like the intensity of the game, and I try to go nuts in the game and score the high as I can no matter what, reaching Phase Level 21 in the game's second stage, that is suitable for the Demonic difficulty, that is Phase Level 21 fixed.
That means that the game gets too hard for me past the second stage, unless I cool down and try to stay away from the danger and intensity, which tears the game fun to pieces. The Practice Mode won't help because it starts the stage on the minimum Phase Level for the chosen difficulty, that is not my situation. It sounds like a very personal issue but a lot of my friends have the same problem. If you are remarkably good at shmups and can surpass it just ignore what I just wrote and this game is a round 10/10 for you. Of course, I'll get good in time and be able to survive Phase Level 21, but that will take a long, hard time, that would turn the game down for some people.
EDIT: I'm not sure if I was clear enough, but I'm blaming the game for being unbalanced, not for being difficult.