Wow--just watched Keroro movie 2, and the easiest way to describe it is to say it's 77 minutes of everything that makes Keroro Gunso awesome. Natsumi reaches truly epic levels of moe-ness, Giroro gets the "twirled around by the evil magistrate pulling on the obi of his kimono" treatment, there's action, drama, family, friendship, afros, even Moa-chan unleashing Armageddon twice. The only thing that could have made it even better is if they'd somehow worked in Giroro being forced to say "Natsumi, my love--kiss me tender, and hold me tight forever" like in episode 38. Giroro is forever the king of GAR in my book for being able to say that twice without dying of embarrassment on the spot.
The plot revolves around two new aliens, Meru and Maru, who kidnap Natsumi to be their "princess". There aren't really any "bad guys" in this to speak of--while Natsumi does get kidnapped, the kidnappers are really just kids, and at first she ends up being more like a mother/babysitter to them. The core cast is all involved, and get at least some screentime--although Natsumi is definitely the star of the movie, which is a definite plus in my book.
With movies based on a TV series the most common complaint I hear is "it just felt like a really long TV episode", but I think this avoided that fate. Scenes like the creation of the underwater city, or the swim Meru and Natsumi take in the ocean, just had that cinematic feel to them where you just look at it and part of your brain can't help going "wow". I think Deep Sea Princess succeeded much better on this note than the first Keroro movie did.
I do feel a little bad not waiting for ADV to release this before watching it, but since they have yet to release even volume 1, the smart money says I'll be wearing adult diapers and re-living 1977 by the time they actually do get around to putting this on DVD. But assuming I'm still alive and mentally competent by the time it hits R1 DVD, this is definitely getting a place on my bookshelf. I'd highly recommend it for any Keroro fans--although if you're not familiar with Keroro Gunso, this probably isn't the best movie to start with, since it doesn't do much to introduce the characters.