The story is pretty basic, there are beings called shadow angels who have emerged for the first time in twelve thousand years to harvest energy from humans. The events of the two time periods are connected but I can't explain exactly how without giving spoilers. Most of the episodes have very little to do with the overarching story. The events of most episodes follow a basic pattern. Agents of the shadow angels appear, this leads to a giant robot fight in which Aquarion randomly develops a technique to gain victory and said technique is never used again. Honestly, it's like they're just making things up as they go with Aquarion's techniques. On a positive note, there are some funny moments throughout the series, but they're few in number. In some ways it does remind me of Bakuretsu Tenshi since it also had episodes that barely connected to the main plot. The big difference is that the episodes in Bakuretsu Tenshi had a lot more variety as to the type of events and episode structures. Still, Bakuretsu Tenshi had a pretty weak story, like Aquarion, but was saved by awesome characters.
Let's take a look at Aquarion's characters to see if the comparison with Bakuretsu Tenshi continues, if Aquarion is another anime where awesome characters carry the series in spite of a weak story. That isn't the case. Whereas Bakuretsu Tenshi's characters develop strong relationships with one another and grow as characters, Aquarion's cast has very minor developments, all of which are very predictable. You might argue that this is a result of having more characters to work with, but this doesn't just hold true with the supporting characters, it's also the case with the major characters and frankly, they could've easily had some character development if they'd spent less time with pointless action sequences. Some of the characters could've been interesting but they don't develop enough to reach that point.
The art is decent, but it's nothing particularly special. The action sequences are pretty creative and intense but there's not a whole lot that differentiates between Aquarion's style and the art style of dozens of other anime. The only thing that really catches the eye about Aquarion's art are the shadow angels since they have some nice, unique looks but the other characters all look vaguely familiar, as though I've seen them elsewhere.
Let's move on to the voice acting. In spite of having some really strong voice actors, Paku Romi, Sugita Tomokazu and Kobayashi Sanae being three obvious examples, the voice acting in Aquarion isn't anything special. This isn't to say that it's poorly done or that anyone errs. It's competent but it doesn't stand out. The music is decent.
The yuri factor is a 3/10. Tsugumi and Reika are pretty homo-erotic but the series never goes anywhere with it except for one throwaway line.
My final rating for Sousei no Aquarion is a 5/10. It's a mediocre series. There are some interesting ideas and there's a lot of potential, for both story and characters, but they never do anything with it and it ends up going to waste. If you're a major fan of Macross and other old mecha series you'll probably get some warm nostalgic sensations from it, but otherwise it's probably not going to hold much appeal for you.
