Summary: I like to play all or most of the way through a game before judging it because you never know how the later levels are going to be. I beat the entire game (39 levels) and can tell you that it needed more variety to make it fun. There is only one really good ship in the game so the decent selection of aircraft to choose from is pretty pointless. You use your ship to do the same few things over and over again.
Summary: I'm considering a purchase of a used copy of the game after renting it. The gameplay is fast and fun and the difficulty can be tough in the beginning until you get used to the controls and what weapons work well for your particular style. The missions are varied early on but can feel repetitive later on in the game. My biggest issue with the game is that for a game called "SeaBlade" there isn't much to do underwater.
Summary: I just bought this game a few days ago and have to say that it's one of the most fun shooters I have played in a long time. Great graphics, tight controls, and lots & lots of action. The difficulty is a little hard, so you might want to start on the "easy" level until you get used to it. But I highly recommend Seablade to anyone who loves arcade shooters!