Not a bad story at all, but it suffered from the same problem I've had with nearly all of these stories - a lengthy, relatively detailed introduction followed by the middle part (where the actual plot happens) that is hurried and not nearly meaty enough, and a short, rushed ending.
I wasn't initially happy to find out Nine wouldn't have Rose as his companion in this one, both because I liked Rose a lot better with the Ninth Doctor than with Ten and because I expect this means she'll be in Ten's story instead of Martha or Donna (both of whom I liked much better in Ten's era), and, well, Nine & Rose just seem like an inseparable package deal.
That said, and slight disappointment aside, I actually ended up liking the way this was handled. It was nice to get an alien as a companion even though she thought and acted a bit too human-like for me to suspend disbelief entirely - I assume it was part of the whole reveal, to make Ali sound like a regular human girl to start with and then only gradually reveal that she's not from Earth and then, in a way clearly designed to surprise the reader, describe her very much alien looks and beliefs. And also, it fit into the canon TV timeline better than I'd expected, and Rose was certainly present enough in spirit, if not in body.
The plot itself... not the worst I've read and not the best I've read. Ancient Babylon was a cool destination, but I felt the Starman part of the plot was rather weak.
On the plus side, I could actually hear Christopher Eccleston's voice for Nine's dialogue, so at least to me, Higson got Nine's voice down just right.