All in all, Parasite Eve is one of the best RPGs around when it was released. The battle system in this game is definitely unique and the one and only. I admit the gameplay is quite hard when you come to face the horrible Mitochondria bosses. But no pain no gain, you won't get to see the rest of the story if you don't defeat them.
This game could be considered as unique with it's battle systems and weapons/armour upgrade. Bonus points earned can be used to pump up whatever you like and it really is worth it. Tools are used to upgrade weapons as well.
After you finish the game for the 1st round, you can replay it in the EX mode with your high levels and weapons. I suggest you replay it in order to get the REAL ending.