The tie-in novels for Doctor Who can be hit and miss. They're quick reads - at about 230 pages and in a larger font, it usually only takes about 2 hours to read start to finish. They're a fun escape on a boring work lunch. However, at $11.99 (suggested retail) they can be a bit of a monetary commitment that can go sour if the story turns out mediocre.
The story here does have a slight juvenile bent to it, simply because of the nature of the pirates and some of characterization and comedic accents. However, I found these touches to be charming and not too silly. The plot itself moves along quickly, with a central mystery that keeps things interesting. It's fairly clever (though don't think about it TOO much or holes will start to appear) without being too convoluted.
The characterization also really shines, and Martha and the Doctor feel right and in line with their on-screen personas.