It is basically a modernized galaga remake.

And it's good even for experienced than beginners (on solo or coop). For the coop, I played with some people wich never tryed this genre... and they loved it ! The coop is very smart, enabling a beginner and an experienced player to have fun together.

Hope it will come on GOG one day ! (I'm really looking for it)

Well, since others have already broken the "vertical" rule I'll chime in with some of my favorite non-vertical shoot'em ups.

First is Syder Arcade, it's a horizontal shooter but you can flip your ship around and go in either direction. This is helpful as you will constantly be getting attacked on both flanks at once. It's fast pace and a lot of fun, you can even set the grphics to emulate old Apple, Tandy or Hurcules graphics which is kinda cool.

I just recently discovered a little indie gem called Retrobooster. It is an awesome combination of Cave Flyer and Shoot'em Up. The way the ship handles takes a little getting used to because the physics are very realistic. The guy who made it used to program simulations for NASA. Once you get a hang of flying the ship the game is a blast. The level designs and power ups are very well done. The only downside was that I played Retrobooster for about two weeks straight, then fired up a game of Raiden and found it a little boring by comparison. The game is not free but it does have a demo.

here's some freeware ones:
http://www.hikware.com/Prod/index.html#wf
http://www004.upp.so-net.ne.jp/x_xgameroom/Works/works.html
[url=http://www.asahi-net.or.jp/~cs8k-cyu/games]http://www.asahi-net.or.jp/~cs8k-cyu/games[/url]/
[url=http://www2.tky.3web.ne.jp/~yosshin/my_works/download.html]http://www2.tky.3web.ne.jp/~yosshin/my_works/download.html[/url]

Operation Carnage is still the best in this regard, and also freeware, not sure where you can get it tho.