Metro Girl is a comic crime novel in the vein of Evanovich's Stephanie Plum series. Since it is a popular fiction genre, there is no critical opinion available. There are only various reviews with critical commentary available.
Although it got generally good reviews, it was noted for being very similar to her other books, possibly to its detriment. This is the main criticism; Evanovich has a specific writing and story style and doesn't deviate from it much, so elements of the story and characterization are very similar to her previous series. Janet Maslin, writing for the New York Times, mentions these similarities as well as her suspicion that the male lead, "NASCAR Guy," feels constructed rather than organic; she also remarks:
This book's fluff factor... [is] jarringly at odds with the Cuban Missile Crisis memories that fuel the plot.
Her implication is that the book might be too light-hearted -- an Evanovich staple -- to deal with heavier topics. The writing is deliberately light and easy to read; this can be seen in the narrator's internal monologue, which helps to set the tone and to inform the reader about her thought processes:
Maybe you don't want to get the police involved right away.
Good grief, the voice of reason said.
Shut up, or I'll bitch slap you into tomorrow, the sister voice [replied].
(Evanovich, Metro Girl, Google Books)
This sort of excerpt is another area of possible criticism; the Publisher's Weekly review comments that "plot takes a back seat to riffs, roughups and dialogue," which could turn off some readers if they find the material to be too shallow. BookReporter.com mentions the "kinky bad guys and quirky good guys [that] only Evanovich can devise," which is a draw for fans, but might also be too niche for new readers -- that is, too wedded to Evanovich's typical style. However, all reviews consistently mention the fast pace, the memorable characters, and the simple amount of "fun" that permeates the pages.
All in all, most reviews of the bok are positive; there is little indication of total dislike or even tepidity; reviewers as a whole (backed up by reader reviews, which are not authoritative) seemed to see Metro Girl as a solid start to a new series.
However, almost none of these reviews touch on literary or thematic qualities of the book. Since it is intended as light reading and not "serious" literature in the vein of Pulitzer- or Nobel-bait, the book is an exercise in adventure and humor, not of deep themes or messages. There is little in the way of symbolism or metaphor; things happen on the page as they happen in real life, and the characters are not internally changed by events or epiphanies. Instead, they act almost as self-aware audience surrogates, commenting on ridiculous situations and providing a barrage of funny banter, sometimes while in dangerous situations that should be taken more seriously. The failure to provide deeper literary meaning is not necessarily a criticism, but it makes criticism more difficult; characters are not allegories for the human condition, for example, but instead are deliberately quirky for quirk's own sake.
Enjoyment of the novel would, then, depend on the reader's own preference. If seeking deeper fare, this book will not satisfy, but for casual reading, Evanovich is skilled enough to entertain.