IKKI also publishes the work of Daisuke Igarashi (五十嵐大介) who I'm a big fan of too. Both Iragashi and Matsumoto have uniquely identifiable artistic styles, and great story lines.
People might be familiar with Taiyou Matsumoto's earlier work Tekkonkinkreet (鉄コン筋クリート). The manga was published in 1993-94 and reached a much wider audience with the 2006 animated adaptation. Directed by Michael Arias, and made by Studio 4°C. It was the first anime feature film directed by a non-Japanese director, and won the 2008 Japanese Academy Award for Animation.
There are a lot of similarities between Tekkonkinkreet and Sunny. Both focus on the relationship of imaginative young boys. Tekkonkinkreet has the street-wise Kuro/Black (クロ) and the simple-minded Shiro/White (シロ). Sunny features the smart beyond his years, aggressive, easily provoked, Haruo (晴男) and his simple sidekick Junsuke (純助). The manga's title Sunny refers to an old broken down Nissan Sunny that rest on their front lawn that the boys use to play and travel where ever their imagination takes them. As with most of Matsumoto's work, the characters speak with strong Kansai accents, which can be hard to follow if you've never read/heard much colloquial Japanese.