I love Lupercal's reviews, but we are often of the opposite opinion. I adore Excel Saga! It's my second-favorite anime. I have not watched any of the animes they parody, but I still find it hysterical. (Unlike FLCL, which I found so disjointed and bizarre I couldn't watch past the first episode!)
Excel is the minion of Lord Ilpalazzo, an ideological maniac who intends to take over the world, starting with F City. His plan consists of sending Excel and her on-the-point-of-death sidekick Hyatt to carry out unexplained and ill-considered missions.Their team also includes a small and cute dog named Menchi, whose contribution to the team is to be an unwilling emergency food supply.
Opposing Ilpalazzo is Doctor Kabapu, an equally bizarre city leader, who gathers up a team of incompetent, squabbling, self-absorbed nobodies to oppose the enemy forces. Half the fun of the series is watching to see which side of the conflict reacts more incompetently.
Though it helps to watch the series in order, it is not a continued narrative. Many episodes are stand-alone, and only require a general idea of who's who to enjoy the off-the-wall humor. There are parodies of every manga genre.
The writer of the manga refused to tell the animators how the story ends, though, so for the last three episodes, the anime creators made up their own ending. It's satisfying, but completely at a tangent from the manga. Alternate universe. And you really need to have seen several parts of the series to understand what's going on.
If you like strange characters, have a warped sense of humor, and think it's funny for a dog to be constantly in fear of being eaten, for a villainous toilet cleaner to be the story protagonist, and for a sidekick to drop dead repetitively at inconvenient moments, well, you've found your series.
One note, though--on the final disc is an extra episode, made mainly as a prank/DVD add-on. Its goal is to violate the bounds of good taste in all ways. I wouldn't say it's not worth watching--there are still some cool parts, like the Excel Musical--but if you're sensitive to extreme bad taste, avoid Episode 26.