Underrated shoot'em'ups

Today I was taking a look at my Dreamcast games and came across Zero Gunner 2 again.
For anyone unaware of this game, it's pretty much your basic shmup using helicopters by Psikyo, but with a nifty gimmick: you can change the direction you're facing, much like the PC "Warning Forever".
The first Zero Gunner is apparently emulated in MAME, but I've never played it.
I never really heard much talk about either of them, didn't even heard of the series until 2 came out. It's rather nice and only moderately difficult on normal (with 8 difficulty settings I beleive).

Anyone else familiar with this game or the series?

If anyone else has some shmups they feel are underrated and not talked about enough, feel free to post about them!

duder, ikaruga isn't especially underrated. esp rade is an underrated game. ikaruga is, in my oppinion very widely known and appreciated by a whole bunch of people, not only fans and wimps.

i do agree with you that it's one of the hardest game ever... if you don't count viewtiful joe, that is.