This has been my favorite shoot-em-up since i've first discovered it. It combines everything I love about the genre into one nearly perfect package. Really the best way I can describe it is that it's one the coolest games I've ever played. It's an epic journey all-throughout.
Aiming your lock-on lasers onto enemies is always fun and very satisfying. This is the only shmup where I switch characters all the time, since I like all the targetting systems.
The atmosphere in Souky also really appeals to me. It's got a very serious tone to it, but it never feels like it clashes with the 'silly' contents of the game (a tank with two dragon heads that move like arms for example). Another review here also mentioned it, but it deserved to be repeated: the soundtrack of Souky is absolutely stunning.
You can tell they were almost trying to tell a story throughout the game, and you can really feel this in the stages. Stage 3 is the usual example, where you slowly descend from the sky and in the background on the ground you can see all the enemies you're about to fight. The music changes each time you've cleared an obstacle of sorts and it's just such a cool feeling.
Really the only complaint I could think of is that you can't really use your normal shot for most of the game if that is what you'd prefer to use. Getting power-ups massively increases the difficulty of the game, so you really want to use your lock-on attacks for damage. I never really minded this since I'll use those attacks for 99% of the game anyway since they are much more fun to use than your basic boring shots.