Definitely one of the best Giant Robot OVAs that I've seen. Interesting, well-developed characters, good story with plenty of twists and enough foreshadowing so they don't come entirely out of left field, amazing orchestral score, and well done action scenes. Only major complaint I have is the lack of a sequel, I doubt Imagawa can be held fully accountable for that.
IMO, it's a definite must buy, especially when Rightstuf has the entire series (plus the GinRei Specials) on sale for $11.