Narue no Sekai is a lot like if To Love Ru had less fan-service, a better romance, and a satisfying conclusion (not to mention an older style of artwork).
Story: The premise is basically the same: nerdy boy meets pretty alien girl, and he ends up getting involved with lots of other pretty girls, most of whom are aliens, robots, or some other such thing.
The key difference, however, is that the boy here (Kazuto) is head-over heals with the alien (Narue) from the beginning, and the two remain a couple for the entire manga. So instead of having a "harem plan", the
other cute alien girls mostly exist to provide fan-service and to move the plot along.
The plot gets overly convoluted towards the end, but is otherwise very satisfying.
Art: Fifteen years ago, I would have said the art was fantastic. Now it looks a bit dated, but if you like the style, it's top-notch.
Characters: Each of the characters is great, and their relationships with each other are fascinating. The only problem, at all, with the characters here is that there are too many of them, and it becomes hard to keep track of them all.
Enjoyment: One of the most enjoyable manga I've read in awhile. I had trouble putting it down.
Overall: An excellent manga, if you like shounen romantic comedies with a sci-fi twist to them. For any fan of To Love Ru or Jitsu wa Watashi wa, give this a try.