If you like analyzing anime, this show will make your eyes pop out. It's the anime equivalent of a Rubik's Cube, you could completely grasp the show, twist it, turn it upside down, yet never truly reach the correct answer.
The short version is, I have to second Zenigame's statement. It's a decent show, I don't regret watching it, but it could have been a lot better with the material presented. This applies to the anime, I'll get to the OVA's later.
The series is divided into three sections, basically - the first ten episodes are like introduction, the following ten are filler/training episodes that are actually fun/entertaining, and the last six are resolution.
I hated most of the young characters in the beginning, all of them had this really hedonistic/selfish/privileged air about them, or had some annoying fixation or flaw that was glaringly apparent. Even Apollo, the best character of the lot, was fairly limited as a character. A fudgeload of themes/ideas/metaphors/references were introduced in this 10 episodes, some of which were dropped/ignored by the end to focus on developing a few core ideas. I guess that was OK, although the dropped ideas I really liked a lot.
Everyone improved over the 10 episode filler stretch, though. That was the purpose of it and it worked well. The last 6 episodes showed how holes in that 10 episode stretch, i.e. characters who had flaws but didn't fix their problems during that time,
The last six episodes were fairly powerful, since with the weight of the 10 filler episodes you could literally see how the weaknesses in the character relationships, humanity as a whole's flaws and the Shadow Angel's own flaws eroded the Tree of Life. I didn't like the development, and can't say I liked the ending and its implications, but it wasn't completely outlandish.
As far as the OVA goes, the first OVA is a ball-buster! I don't think the magnitude of how awesome this OVA is would be apparent to someone who didn't see the series first. It was as if someone air-lifted the fully developed characters from the anime and simply restarted the conflict. A few characters were killed in the interim, but that just improved everyone else. The backstories are slightly different, but they're done in a tasteful way and how the characters changed as a result of these differences is immediately apparent.
Apollo, for example, was awesome in the series. He's attained Gar status as of the first OVA. He's intelligent and well-learned (there's a library in his cave) despite being a mountain man, he's still got his heightened senses and is equally as protective, but he's matured a lot. He also wears a kickarse poncho that makes him look all the more mysterious.
Sylvia is better too, while Lihuha loses her annoying "I'm so unfortunate" motif and gains a new reason to be depressed, which was a lot better IMV.
The second OVA is a disappointment, especially given
was apparently misleading promotion for it, and feels like the production was rushed. The main idea was expanded, but a lot of weird unexplained things happened, the development was more coarse and the ending was a major WTF. Not Evangelion-style, overall this ending was a lot better than the first one, but people still died, just a different set of people. I put it at the same level of quality as the series, maybe a tad lower because SO MUCH was crammed into so little time, it wasn't very coherent.
Overall? Worth watching once. You will think about this show after seeing it. There's so much to debate and analyze I was overwhelmed and didn't even try.
The fights that Zenigame talked about were kind of mindless to me, they were rather absurd super robot fights sure, but the exaggerated silliness wasn't a point of contention for me.