The pictures are vibrant and goofy, with fly guy's big eyes and enormous grin mirrored by Buzz and his family's Captain Crunch eyes (which go over their hair the way that Crunch's are somehow placed on his hat). A lot of emotion is conveyed by very simple lines, and the quality of the illustrations is much higher than on many early reader books.
Because this is an ongoing series, there is ample opportunity for character development. I'm half tempted to go pick up Fly Guy Meets Fly Girl and Fly Guy's Ninja Christmas just for fun! I can see fans of Arnie the Doughnut or Pete the Cat being very happy to get these books as gifts, because this strikes me as the kind of series that children want to collect, starting with Hi! Fly Guy (2005)
I've had a lot of struggling readers this year; Fly Guy would go over well with them, but I think the next too are strictly elementary. Also, I've never been a fan of the whole fairy/princess thing aimed at girls. Picky Reader was 4 when the tend started (2002), and although she got very into Malcolm's The Jewel Kingdom #1 series, there is only one picture of her in a princess costume!