rRootage is a nice way to get into the bullet hell genre. There are levels ranging from reasonably doable to totally impossible, and the difficulty rises gradually, with no real difficulty spikes. I am currently at level 4-C and it is very hard, but still doable.
The nice thing is that you can choose any level out of the bunch, and that every some points you get extra lives, which allows you to stay longer.
The bullet patterns seem to be randomly generated OR taken from a huge base of them, however, they always fit the level difficulty.
Get it here:
http://www.asahi-net.or.jp/~cs8k-cyu/windows/rr_e.html
If you want a few laughs, try playing the very last level and see how quickly you die!
Update: I'm now at 7-A, and it becomes virtually unplayable without the pause button. The last levels seem impossible even with the constant pauses.