Giant Robo should have really sucked. It had a retro-futuristic style (think Astro-Boy), it has some really cliched characters and the story doesn't start out in any exceptional way. Except... as the story progresses, these things start to change. Characters lose their one dimensionality and start to have human motivations. Plot twists drive shake everything the narrative has led you to believe. There are times when it appears the story just can't get any bigger, but it does!
What makes this series great is giant cast of characters (each with unique abilities, some of which are very original), the twisting of convential anime plot lines and that it doesn't rely purely on shock value or huge explosions to keep you interested. Having seen a lot of the MD Geist-style anime (read: very very bad anime in regards to plot or characterisation), it is great to see a series that doesn't insult my intelligence.
There are some negatives to this series - the whiny boy who controls Giant Robo, the voice acting for some of the characters (I saw a dubbed version), there are some really cheesy moments and some anime conventions are still followed religiously (eg the girl (Ginrei sp?) is mostly useless... except when she isn't). A big let-down is the additional episode on the final tape - because there are 7 one-hour episodes, a filler episode called "Barefoot Ginrei" was after the final episode. It doesn't really add anything to the story, and the fact that Ginrei's feet aren't the only things that are bare may have been the reason it was included.
Despite this, Giant Robo deserves any accolades it gets. Go out and see it.