Kanon is the manga adaption of Key's Kanon PC game:
To make this brief, Kanon is about a guy (Aizawa Yuichi) who started living in a town, where he meets lots of chicks and had fun everyday. Every chapter of the manga is dedicated to one chick, where their entire arc from the game is summed up and "manga-nized". This Manga is all joy. There are a lot of cute and funy bits and most of the sad moments are taken out.
The art, while unrealistic, is cute and cuddly. The art is very lively and informal, perfect for leisure reading.
This might be a bit bias since I read this after watching the anime (2006), but I reckon the characters in the manga are perfect. The male lead is kind hearted, funny and mean at times while the female leads are all unique and most importantly, MOE.
Kanon is a well-made comedy manga. While it's drifted from it's romance and harem ideas (game and anime), it's comedy turns itself into something enjoyable.
For people who watched Kanon (2002 0r 2006), the review might make more sense here...
Your Enjoyment of the anime - Sad moments + More kawaii moments + More funny moments= This Manga read more