The illustration style is a little different than some I have seen; a little bit more like a comic book, but not entirely. The font is in all caps, but a very readable san serif font that is not as small as some books. The color pallette is heavy on grays, brown, and greens, befitting the sometimes militaristic aspect of the plot, but does have some pops of color when the mood is being lightened.
Ben abd Erica certainly have the most time on page, and their relationship is noteworthy, but in this version there were a couple of characters that really popped a bit for me; the doddering Professor Crandall who is just pretending to not understand what is going on, Erica's father, who is shown in a sharp suit, and Ben's friend Mike, who hovers on the periphery, trying to understand what is going on with his friend.
This alternates pages intense with word bubbles with pages that are mainly action, so is well paced and easy to get through. There's still a lot of text, so best for fans of wordier graphic novels. This is not quite as believable as Alex Rider or the Gallagher Girl books, since it walked right along the edge of goofy, but seeing the pictures somehow adds to the believability.
I'm a huge fan of Gibbs' series-- Fun Jungle, Moon Base Alpha, and Charlie Thorne, so it makes sense that his works would get a graphic novel treatment, and what better book to start on than Spy School?