Zone of the Enders isn't great. As you said, there's a lot of going back to the same maps. The 2nd Runner is so much better that most people will recommend just skipping the first and playing that one. The combat is better and has more, sub-weapons are actually useful, enemies are smarter and have more variety, the protagonist isn't a whiny kid, and there's no backtracking. However, I suggest playing one of the re-releases, and not the ps2 original. JP and PAL regions got a special edition with a load of extra content, that the U.S. never got. Later releases, like the HD Collection, have the added content from those special editions.