The art style is very old school, and reminds me of Takahashi Rumiko somewhat. Expect lots of overly exaggerated expressions and some super deformation. The style is consistent with the atmosphere of the manga, and never seems too far out of place.
The characters are mostly stereotypical, but all primary characters have some depth. Designs for the main characters are good, and it's easy to find one or two favorites among the bunch. I mostly like Kahm, but all of them are enjoyable in their own ways. Some of the tertiary characters are so generic, you'll quickly forget them or confuse them.
The story is not terribly original. Aliens invade Earth, boy meets alien princess, fights for his life and survives, the two fall madly in love, and everyone in the galaxy tries to split them apart. There are lots of touching moments, and times of crisis to test their resolve.
The plot is decent, and evolves quickly to match the action. As our heroes struggle to stay together, they must overcome conspiracies which have been in place for thousands of years. There are a few dark secrets to be uncovered, but nothing too heavy to ponder about, and everything is fully revealed by the end of the story.
Overall, a fun, ecchi space opera, with lots of action, romance, and a touch of drama. Recommended reading.