The plot of the first season is mostly simplistic - enemies invade from space, conventional weapons don't work and the super robot is all that can fight them effectively. There's a slight plot about the main character's search for a missing sister but really nothing is done with this until the second season. Instead, you get lots of explosions, shouting, nicely animated fights and interesting enemies. Some people might think this too simple, but at 13 episodes it's a lot more manageable than say the 50 of a show like Voltes V or the nearly 100 episodes of Mazinger Z.
The music is consistently great, with JAM Project doing a sterling job with the incredibly catchy opening and action themes. Similarly, the little details both in sound and visual design give Gravion a sense of weight and power that some robots don't have - the way it takes a steady stance and clamps itself down before using a rocket punch is a great detail.
However, the action is only half the appeal of Gravion. The other half is the comedy, which is at least on a par with Full Metal Panic, and to my mind better. It's mostly simple harem stuff like cross-dressing, voyeurism and so on, but it's done well and generally raises a good few laughs each episode. The cast of characters is amusing, from the mostly female robot team to the two male pilots and Sandman, the dramatic operator.
This brings this review quite nicely into the strongest point of Gravion - its English dub. The voice actor for Sandman is excellent, getting the right tone while sounding like he's really enjoying himself.
All that said, it definitely warrants saying that Gravion has elements of the ecchi genre in it as well as the mecha action. The girls are carefully chosen to fit all the core fetishes of a harem show, and most of the humour is sexual. However, it reminded me more of Kotetsushin Jeeg's laddish humour than Godannar's more awkward sexual jokes.
If you don't mind a good amount of risque humour and bouncing boobies in your mecha show, and love JAM Project and oldschool super robot action, then Gravion is well worth watching. The main complaint, though, has to be the reduced significance of the main arc plot in season 1 - it sets itself mostly up for season 2.
