There could be no better way to lift the curtain on Eureka Seven than with a song as beautiful and freewheeling as the show itself.
Completing a neat little run of songs featuring the rock/pop band Flow, this is undoubtedly their most famous contribution to the anime world. Powered by a soaring,surging set of synths, DAYS is pretty much as perfect an opening to Eureka Seven as one could hope for. It’s got the energy and get up and go that the series has in spades, but also the wistful, regretful beauty that showed up in the show’s quieter moments. The song has a wonderful, distant quality while at the same time never forgetting it’s a catchy pop tune. Then there are the visuals – windy hills, blue skies and surfing robots, everything you could ever hope for in an Eureka Seven opening. Simply one of the best ever.
The Glorio Blog 
I’m not a huge fan of this song (or of Flow really), but I can’t deny that the OP sets a great tone for the series and generally fits the show very well. I love Eureka Seven to bits, so even though I didn’t like most of the OP and ED tracks much, I’m glad it got a quality soundtrack. Now composer Sato Naoki, I do adore.
I just feel the surge and spirit of this song fits the show so well that it’s impossible for me to dislike it. Having said that, I’m with you on all the other OPs and EDs, which are alright but not really great (except for ‘Himitsu Kichi’ which we’ve already covered in this feature).
Yeah, Himitsu Kichi is a lovely song, and by far my favourite of all the OP and ED tracks.
I always felt it and ‘Fly Away’ were sequenced the wrong way round, since they use ‘Himitsu Kichi’ while the show’s still mostly a knockabout comedy and then ‘Fly Away’ when stuff gets real dark. It’s jarring.
“the other OPs and EDs, which are alright but not really great”
Lies, every single E7 OP was god tier. You could have featured any single one of them and nobody would have batted an eye.
Days is quite a lovely song, it’s by far my favorite of the Eureka Seven openings and just an awesome song in general. I tend to really enjoy Flow songs so it was no surprise that I really liked this one. It fits the series quite well and I have fond memories with this song since I thoroughly enjoyed Eureka Seven when I watched it a few years ago, though I can’t be too sure which Flow song is my favorite overall.