In a class of its own!!
Basically, you just send messages to and fro between yourself and your local (or online) friends. When they get online, they will get your note and reply fittingly. Eventually, you can customize the way your mail looks and can add things like color, pictures and (if I'm not mistaken) videos. And you can send a chain of messages to people in one message if it's something longer you have to say. And the mail that you get displays itself in a slithering chain in order from oldest at the left to the newest on the right. It's really neat! Besides the eShop, this is probably the best app I have.
Swapnote is truly something remarkable. Right now I don't know anyone with a 3DS besides people I've come to know online. However, it's a blast sending and receiving notes from friends. The other day I sent my friend (and user here), Spinnerweb a message that said that I hoped this was the start of something special now that we were friends on the 3DS. He wrote back something akin to "Yeah.....So you think." and began to draw a devilish character with a pitchfork which I thought was humorous and cute. This thing here will make you smile at times guaranteed. I haven't had the app that long but I totally see its potential. Don't sleep on it!!!
P.S. This review may be a bit biased but only because I was a huge fan of Pictochat.
Final score ------> 10/10
written solely by darksongbird