Soul Blazer's only real noteworthy aspect is being the first part of Enix's Heaven and Earth trilogy. So if it was never released, we might never have seen Illusion of Gaia and Terranigma. If you don't take that into account, however, it rather blows.
The story is simple. You're a messenger from God with a big-ass sword, and you revive the world by going to dungeons and killing bad guys. The dungeons are filled with "monster lairs" that continuously spawn the punks, and once you kill enough of them the lair explodes and something is revived. You have to blow them all up. That's it. I think you can imagine how after a while this gets really, really tedious. Sure you travel to some different locales like ice mountains, oceans, beaches and so forth, but since the point of the game remains absolutely the same, it doesn't really feel like it ever changes. Well, for variety a boss is thrown in once in a while. Expect to get killed a few times, then win with ease as you figure out the mechanical pattern.
There are some little secrets thrown in, but they are really, really little. Soul Blazer has none of Terranigma's epic feel and none of Illusion of Gaia's uniqueness. It's extremely bland, from the monotonous gameplay to your own, completely faceless, personality-less hero. I suppose you can play it if you liked the other two games and wanted to find out where it all started (and it is true that Soul Blazer has a sort of ridiculous appeal), but there's no other reason. Terranigma still has a loyal, if small online community dedicated to it, but somehow no one really obsesses over Soul Blazer anymore. Well, it's for a good reason. Don't waste your time.