I got this as a Christmas present several years ago. I hadn't played it in a long time so I tried it again today. It's a classic! While it has primitive 3D graphics compared to the PS3 and Xbox 360, this is one of the best games available for PS1.
There are many different levels all in 3 main worlds, each with its own boss. Their are countless mini-missions in each level, and all work toward finishing a world and defeating the boss. The system Insominac made is a very good solid one that keeps you wanting to win without getting bored.
The levels, worlds and bosses are all very unique, creating a fun, entertaining game. While the audio and graphics aren't half as good as newer consoles' games, the content is much better in some ways.
I recommend this game to anyone with a PS1 wanting a new, fun game. Check it out!