If You Liked Lucky Star and Haruhi...
...because that's what it is. That's what Haruhi-chan is. It's Lucky Star with the characters from Suzumiya Haruhi no Yuutsu. 3-minute sketches, occasionally utterly bizaare things happening for no apparent reason, incredible amounts of in-jokes. Some of it will still be hilarious to people who have never seen Suzumiya Haruhi no Yuutsu - such as the scene where two of Koizumi's helpers show up in a highly improbable and bizaare fashion and state dramatically "I WAS MERELY PASSING BY COMPLETE COINCIDENCE" - but I think a lot of the jokes rely on pre-expectation and foreknowledge of characters. Luckily, as stated before, it's just as trope-subversive as its parent series, meaning that if you're media-savvy you'll probably know what some of their things are subverting. I would hesitate to say that it's 'just for fans' in the same way that Lucky Star was not 'just for Otaku', but Suzumiya fans are probably going to enjoy it on more levels than just 'some guy' would.
Wonderful animation and music though, especially once they stopped using their strange 3d engine and went back to traditional animating. For fans, everybody's personalities have been either flanderized or tweaked - Yuki, instead of being a stoic emotionless bookworm, is now a closet otaku, for instance - leading to absolutely hilarious situations. Mikuru is even more of a ditz then you could think, Itsuki has a permenant fox expression, Kyon can't stop being the buttmonkey. Occasionally the art will shift dramatically into the parent show's style, but don't be fooled; it's just a setup for more hilarity.
Anyway, if you liked Lucky Star, or you liked Haruhi, or you liked both of them, then you'll almost certainly enjoy Haruhi-Chan. If you didn't, well... probably not. And if you have no idea, you should probably go watch Haruhi first.