Now we're talking.
More correctly titled Keroro Gunsou (Sgt Frog being the translated manga title), this is the comedy hit of the last 5 years.
And here's why:
Genius steals.
The show Urusei Yatsura, more on which later, has spawned multitudes of imitators, but few have understood what made that show great. Keroro Gunsou, however, gets it.
It hits the correct mix of extra-terrestrial home invasion, mad science, parody and thwarted romance perfectly. And while the anime is a toned down version of the manga, that also works to it's advantage as had it maintained the more ribald nature of the manga it is unlikely to have found the audience it has. If anything the humour is more effective now as it hits on a variety levels, while kids can appreciate the slapstick and cute characters, there's plenty of parody and satire for the parents, and relationship comedy for the teen audience.
And it has a cracking cast. Chiwa Saito plays the female lead Natsumi Hinata, and makes a great straight woman for the rest of the cast. Kumiko Watanabe turns in a wonderful performance as the titular Keroro (I see she is also the voice of the Chapapapa-ing Fukurou from CP9 in One Piece!). And best of all, it has the legendary Jouji Nakata as the lovelorn Giroro.
Now the first 51 episode season is awesome and well worth your time. The episodes fansubbed so far in the second season have been a little below par. While the first season contained stories original to the anime, they tended to be equal to the manga adaptations. The start of season 2 seems to be assuming you've not seen the previous season, and spends a lot of time reintroducing characters. I'm guessing there was either a gap, or a schedule change that necessitated this, but it's a little annoying if you have seen the previous episodes. Clips I've seen of the middle of the second season seem to indicate it picks up again.
The big question is why has it not been released in the west? Bobobo has had a run on Cartoon Network, and that must be a lot harder sell. It has just been shown in Italy so there is some hope of movement there.