The only thing where FLCL stands out is presentation. With it's own fuzzy and wacky style. On the other hand the FLCL is weak in everything else. By "else" I mean content itself: zero character development, weak character back-story, weak and plain idea of the story arc, e.t.c.
You may say: "What kind of "character development" you expected in 6 episode anime?"
Anything but a zero. Even Nichijou (despite being "slice of life" genre) has more character development in 2-3 episodes than FLCL in all 6. (Yes, I know there is 10 years difference in them. Yes, I know FLCL has it's place somewhere in "classics". No, that doesn't take away all the Story, Character flaws FLCL has.)
It's like me carrying a video camera around filming random stuff than I do, the jokes I said, the funny and stupid things that I did,... with a nice, fitting soundtracks being played when needed. Would you call it a masterpiece? (don't answer.)
Outro:
If you are looking for something to take a break from "convenient" and "stereotypical" anime then this might work for you. FLCL is a wacky mixture of "slice of life", "Mystery" and "action". If you don't mind zero character development and plain story - go for it.
And if you are still in doubts and keep reading other reviews for answers - just stop and watch anime already. You will spend less time watching actual 6 episodes long anime rather than reading all those essay reviews.
