Sometimes anime is truly incredible, like a gift sent down from On High by the very gods themselves, and sometimes it's so bad that I'm amazed the fabric of the universe doesn't rend itself apart just to swallow it whole and forever spare mankind from its sight. Shichinin no Nana is somewhere in between--I can't say it's bad because there were a few moments that were funny, but I can't say it's good either because I seriously doubt I'd watch it again.
Shichinin no Nana pulled me in with an interesting premise--junior high student Suzuki Nana is studying for her entrance exams for high school, living with her inventor grandfather while her parents are working abroad in America. She has a crush on a classmate, Yuichi (no, not the pimpmaster from Kanon) who likes taking pictures, but she can't get the guts up to kokuhaku (confess her love) to him. Her supportive best friend, Hitomi, who IMHO is the best character in the series but sadly underutilized, encourages her to make chocolate for him for Valentine's Day. However, when she goes to make chocolate for him, she finds the microwave has been moved to her grandfather's lab, which is filled with useful inventions like a VCR toaster. She opens the microwave, and there's a strange crystal inside that's activated--in a blinding flash Nana is turned into seven, the original plus six copies that each have a different personality trait (easy-going, crybaby, angry, etc.). Now her grandfather's food bill's going up by seven times, not to mention they have to decide which Nana gets to go to school, and since they all have the same crush on Yuichi, they ALL want to go.
* some spoilers to follow *
There are a couple good points about the series: the animation quality isn't bad, and the original idea has a lot of potential. As much as sometimes it seems formulaic, there are a few parts where it breaks out of the mold: for example, Nana's crush, Yuichi, isn't just sitting around waiting for her to kokuhaku--he actually likes someone else. And the appearance of "Black Nana", the physical form of Nana's negative side if you will, towards the end makes for an interesting antagonist. The depiction of the life of a student desperately cramming for exams, knowing she might not make it, was very well-done too, and probably a more realistic portrayal than most anime where school is a kind of laid-back utopia. Plus Nana lives in Kyoto, which my wife and I have been lucky enough to visit a couple times now, and is easily my favorite city on Earth. The background music is also good--unique, but it fits the series well.
Then there are the bad points.
A few episodes in, it feels like the writers are already running out of ideas, and the original Nana plus the other six end up using the superhuman powers they get from the crystals, putting on "Nana Rangers" uniforms and forming their own sentai team. This is where it started going downhill for me. Hitomi, Nana's best friend and the only one who knows their secret, is an interesting character but her screentime's not all that high, and only one episode really put her in the limelight, which is kind of a waste. The "san-nin-gumi" of girls who sort-of bully Nana all talk at once, as do all seven Nanas half the time. That may not sound annoying, but believe me, it is. Yes, they're all copies of sorts of the original Nana, but the 98th time they all say the same thing in seven-fold stereo my ears start bleeding and it's hard to resist the urge to take the DVDs out and go skeet shooting with them. Yuichi also gets very little character development--he's kind of just there. The vice principal is just a walking stereotype, which makes Hitomi liking him very, very wrong. And the ending is...probably the worst ending I've seen since Maburaho.
Shichinin no Nana was licensed by Media Blasters in region 1 as "Seven of Seven" (Shichi-nin = 7 people, and nana is also "seven" in Japanese), in spite of the fact only the first two eps as I recall were ever fansubbed--but this was back in the anime boom years, and maybe they got the license really cheap. In the end, for me this just didn't live up to its potential, but your mileage may vary.