I'd say it's a bit in between. It's definitely not as "lighthearted" as Den-O, but at the same time, not as serious as Kabuto. Granted, it's only been 2 eps, so it's still hard and a bit early to make any real judgements. There are definitely questions/mysteries that are going to need to be explored regarding the main character, Wataru.
Only definite thing about it that I could say is that paying attention would be a good idea, since the switching back and forth from 1986 to 2008 might otherwise get really confusing.
EDIT: First teaser/preview of the Gekiranger VS Boukenger Special!
Props to the HJU peeps for finding it!

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