Vertical Bullethell shooting games are a big thing. You've seen tons of them at the arcades, the japanese gaming industry pumping out thousands of them, even indie developers like ZUN making ENTIRE SERIES of them. The thing is, that's because they are brutally challenging, aesthetically pleasing, and reward high risk gameplay and daredevil flying, so a lot of skill and high reactions are required. Danmaku Unlimited 2 is perhaps one of the purest examples of such a thing. It's the Bullethell shooter genre distilled into a two-flavoured package.
I say that because DU2 actually has two modes of play. Both are exactly the same game, and the upgrades and progressing you unlock (oh yes there's that too) apply to both modes, but each mode changes the core mechanics of the gameplay.
It's not immediately obvious from the outset, so here's a rundown. You have Burst mode, and Classic mode to choose from when you start a game. Classic Mode is your average Bullet-hell style shooter. You have a ship that's pretty mobile, it has a tiny little hitbox (that nice glowing core on your ship, don't let bullets touch it!), and you can shoot your guns. Your primary fire (Z, or X on a 360 pad) is your wide area rapid fire shooting, and your secondary fire (X, or A on 360 pad) is your Focus ability, which most STG fans will know is where your fire goes to a narrow path straight ahead and is concentrated, all point items on screen are sucked towards you, and movement is slowed allowing for more precision movement. This allows you to weave in and out of the beautiful bullet patterns to "Graze" your ship along bullets. Grazing increases your score multiplier and also powers up your "Trance" ability (C, or Right Bumper on 360 pad), which is the same between both modes - a quick wipe of all bullets CURRENTLY on the screen, and your power is massively increased, allowing you to destroy enemies rapidly, also causing them to drop very high value point items... for a limited time of course. Once Trance runs out, it has to recharge. In Classic mode, it automatically recharges over time.
In BURST mode on the other hand, you still keep your regular primary fire as normal, but your Focus ability is changed to a "Beam" ability. It has all the benefits of the usual Focus mode, EXCEPT that you don't fire again until AFTER you release the button, and ONLY if your Beam gauge is full, sort of like a mini Trance power! The Beam power is STUPIDLY powerful, and wipes out bullets it shoots too, so FANTASTIC for getting out of a tight spot. However you can only actually fire with it if the gauge has enough power in it, and hte more power, the longer it fires for. How do you build up the guage for the beam? You collect point items! Simple yes? Trance is exactly the same in BURST mode, except that in THIS instance you can ONLY build your Trance gauge up if you graze bullets. It does not automatically refill this time. You actively have to put yourself at risk to get it back so you can save yourself in a pinch.
Here's where Danmaku Unlimited 2 really shines. At its core it's pretty bog-standard STG format, with a few little twists (like Burst Mode for example), but it is a game entirely built around risk and reward, ESPECIALLY with the Burst Mode. The closer you are to enemies when you destroy them, the more your multiplier is built up, and the higher value pointitems they drop. The more bullets you graze, the higher your score, the quicker you get back your Trance gauge. The better you do and the further you get, the more 'XP' you get, which grants you more points to spend on upgrades to make your ship ever more powerful.
It's devilishly hard, and a lot of fun... and that's not even mentioning the ASTOUNDING soundtrack by BLANKFIELD. Give it a listen, it's pulse-pounding, adrenaline-fuelled metal and it is GLORIOUS. It's just a terrible shame it's so pricey at $9.99!.. but then you do get the soundtrack for the previous game in the series, the sadly iPhone only "Storm Strikers". Oh well!
(I think there's a Danmaku Unlimited 1 as well, but that's again iPhone only... and may actually be Storm Strikers, I don't know. I can find VERY little details on both Storm Strikers and Danmaku Unlimited 1!)