The subject at hand, intersexuals, is an interesting topic. I believe that this will be a very satisfying read for most everybody. The first volume is two stories about two I.S. while from volume 2 through 8, it is about the life of an I.S. named Haru and the individuals around him.
The mangaka does a great job fleshing out the story, and I grew attached to the characters. The plot is engaging and solid (though some people will probably find the character-with-conflict, solve conflict, jump to another character-with-conflict, solve, etc. a little dull). There's a lot of deeper meaning behind the actions of the characters, and for me, this has made me want to cry more than a few times. The characters really grow, and the struggle to rise above society's either-or notion for gender is well-thought out.
The art is a little simplistic compared to some other manga but I find it refreshing and fitting for the manga. This all depends on your personal preference, of course; just browse through a few pages to see.
I.S. is a very realistic read; it's easy to imagine this happening in real life. If you're looking for a good deep read regarding a group of individuals that are the most overlooked in our society, this is for you. The manga carries a very strong message and tells it beautifully. If you're looking for a light shoujo, I say it's probably a waste of your time.
I'm a little disappointed with JanimeS decision to stop scanlating after volume 8, and all I can hope for is a group to pick this back up. Nonetheless, I highly recommend this.