Salve Corneli!
To quote Walter in the Big Lebowski: you're entering a world of pain.
No, seriously, my comments... Well, I can give them both from a philosophical point of view and a literary point of view.
In terms of philosophy, the book is okay. I remember that when I read it (I was relatively un-versed in philosophy), I was able to understand most of it. The trick however is to remember it, and that's a tougher job, as with any philosophical overview. In this respect, and in terms of difficulty, it's not radically different from most other broad chronological overviews. The reason why it's labelled as accessible is because it's been wrapped in an understandable story.
Now as for the story, I do have gripes with the book. Most people I know who have read it thought it to be, at many points, too repetitive and boring. It's one of the few texts which has been able to put me to sleep (I'm not joking). Expect no humour from the book. If you want to have a good laugh, I would recommend Luciano De Crescenzo's books on ancient philosophy in a modern world. They also have a story between the lines and are both entertaining and educational (he only writes about ancient philosophy, however, I believe Gaardner covers more).
In short: the book is ok. But it's not a page-turner and has a deceptively easy image.
Vale optime!
Draco
Gn. Dionysius Draco Invictus