My reasons for finding Raideen to be a mixed bag come from the mixture of what it does right and wrong with its series elements, and I'll elaborate on what it does right first. For a refreshing twist to the mecha genre, Junki isn't portrayed as an unstoppable pilot when utilizing the Reideen as he finds himself increasingly challenged throughout the series by the alien threat as they continually change their tactics when trying to lure out and capture the mecha with the alien threats unleashed mostly taking more than one episode to resolve. In addition, Junki finds himself conflicted throughout a good chunk of the series with what role he should have in the conflict surrounding Reideen and eventually comes to terms with what he wishes to gain out of it and hints are dropped with some of the characters having bigger roles in the conflict than one would assume on the surface. Also, the presentation to the series is great for the most part with excellent CG animation work on the mecha and alien weaponry, fluid mecha battle scenes against the alien threat and a good number of tense musical tracks with Reideen's soundtrack that are fitting for the show's serious mood.
What greatly hurts the series are a number of significant flaws that it has in regards to its storytelling and depth. The series quite often relies on deus ex machina for Junki to get out of any major crisis with the Reideen whenever it seems like he is about to be finished off and this results in a rather poor ending for the series when this lazy storytelling device gets milked to the umpteenth degree. Also, the series does a poor job of fleshing out and focusing on a number of major story elements and characters critical to its plot. The alien Roxell is introduced as a rival for Junki seeking Reideen for his own ends in the middle of the show's run, yet doesn't show up again until near the end of the series and his motives don't get much fleshing out. The mysterious transfer student Midoriko is hinted to have a connection of some sort to Reideen, yet the series never bothers delving into great depth with it. Also, the aliens attacking Earth to claim Reideen also lack depth in their motives and where they come from.
Pretty much, Reideen's mixed bag status stems from what it does right and wrong with its mecha storyline. While it offers a great presentation and some solid twists to your typical mecha storyline, it suffers a number of major storytelling flaws due to dabbling in deus ex machina and not fleshing out a number of its major series elements. Your mileage will certainly vary with how you warm up to this modern retelling of the 1970s mecha anime classic.
