The Composition notebook is a softcover notebook that's a bit larger than the standard A5 notebooks they offer. Leuchtturm calls it a Slim B5, and that's a perfect description. It's a 10" x 7" notebook with all the niceties you've come to expect from Leuchtturm. As far as I can tell, the only thing missing in these notebooks is the inclusion of a "Date" line at the top of each page. Instead, each page is simply full-bleed dots, lines, or plain paper.
The soft cover is also a really nice touch considering the size of this notebook. I can imagine that a hardcover book might be a bit harder to handle and pack than the malleable softcover on the Composition notebook, and I'm already a big fan of the A5 softcover notebooks.
With 121 sheets, there's plenty of space to write, draw, doodle, and dream for quite a while. The paper is the standard 80 gsm that you'll find in any Leuchtturm notebook, and it handles different ink types fairly well. Fountain pens with larger nibs do tend to show through a bit on the back of the page, but it's not so bad as to prevent me from using the back page for most things. As far as bleeding and feathering, I've never had any issues with that regardless of pen and ink combo.