I'd heard that it was nothing special, but I watched it anyway because I wanted to see its sequel, Mari Okada's Aquarion Evol. Unfortunately, everyone's right on this one. It's not unwatchable, but it's not worth hunting down.
It's a giant robot show. Mankind is under attack by Fallen Angels. (Don't believe the official translations that say "Shadow Angels". The word they're saying in Japanese is "datenshi", i.e. fallen angel, devil, Lucifer, Satan.) The only thing that can save us is giant piloted robots that combine into three-in-one merchandising opportunities.
Also, the combining procedure involves all three pilots glowing naked and having an orgasm.
The show has two problems. Firstly, it drags on too long. That was normal back then, mind you. 26 episodes used to be standard for an anime series and you'd expect filler, but even so you could improve this series significantly by cutting 13-14 episodes in the middle. You'd keep the first few episodes and everything from around ep.20 onwards. (Ep.22 is where I stopped wanting to fast-forward through episodes.) You can skip most of what's in between those, though. If you still want to dip into it anyway, perhaps try these:
Ep.4 = is quite cool, showing that a giant robot pilot would need to learn to walk.
Eps.8,15 = playing strongly with the "mecha = sex" metaphor.
Ep.12 = Apollo is ordered not to touch the feather, so he touches it. D'oh. Lots of past life revelations here. Did I mention that the three main characters are reincarnations of people from 12,000 years ago and that everything's happening because of stuff they did then?
Ep.13 = an important death (if you're Apollo).
Ep.16 = one of the cast turns out (not very surprisingly) to be a nice vampire. Bizarrely, nothing is done with this. I was waiting and waiting for the revelation that the vampire was alive 12,000 and was involved in those ancient initiating events, but no. The show just drops it in and then forgets about it. It's mildly amusing to see Sylvia's jealousy when Apollo gets sucked on, though.
Eps.17-19 = it's as if the writers were going stir-crazy waiting for the finale. Ep.17 has the girls deciding that they need to diet and being annoying about it, although it improves in the second half. Ep.18 has characters role-playing as each other, which was sometimes funny. Finally, as a one-off, ep.19 does a completely different art style, as if they'd hired Mick McMahon.
The sex is a bit odd. This isn't the only anime with a "mecha = sex/puberty" metaphor (or more), e.g. Neon Genesis Evangelion, Eureka Seven, Star Driver, Captain Earth, DARLING in the FRANXX and maybe a bit in Gurren Lagann. The naked orgasm transformation sequences still stand out, though, as do the extended metaphors in eps.8,15 and Sylvia being naked throughout the finale. (It's glowing and family-friendly, though, with no nipples.) Oh, and Sylvia thinks her brother is the reincarnation of her lover from 12,000 years ago and doesn't mind at all. In ep.9, she says she'd like to go to bed with him.
That's distinctive... but, to be honest, they don't really push this very far. They're just playing around. I'm expecting more from Mari Okada. They also never address or acknowledge the threesomes! Aquarions need three pilots, which means three people sharing a glowing naked orgasm... but the metaphor always stops there. What happens next is always just a giant robot battle scene.
The other problem is that the cast aren't very interesting. There's a love triangle of sorts between Sylvia, Sirius (her brother) and Apollo, but Sirius is a dull warrior aesthete snob who looks down on everyone. I never really cared about him, even during his dramatic finale arc. Apollo is fun to watch. He moves like a lizard and eats live rats. I also liked Tsugumi. Otherwise, though, the cast are sort of okay. The mission's original commander, Jean-Jerome Jorge, is a mildly tiresome "Captain Wrong", while I couldn't help wondering why Gen Fudou never went on missions if he's so much more strong and experienced than everyone else.
The show's danger never seems particularly dangerous. The Fallen Angels show up occasionally, have a fight and then leave. We know they harvest humans, but this is only demonstrated graphically right at the start of the series. An apocalypse is averted in the last episode, but we'd only learned about it ten minutes earlier.
The show's okay, but a bit meh and not as good as its music. (Watch the scenes where the show tries to be super-cool by using its theme song as incidental music. This can be powerful in shows like PreCure and Cutey Honey, but this show never earns it.) The metaphorical sex and potential incest could have been explosive, but doesn't even try to get there. We're a long, long way from Daimidaler the Sound Robot. It's mediocre.