My reviews have a tendency to elephantiasis, so as a corrective I intend to shrink down what I have to say. Thrice.
The one line review: I have no respect for authors whose works are merely parasitic upon the classics of English Literature.
The comparative review: a retelling of Shelley's Frankenstein that sticks remarkably closely to the original in many regards: Frankenstein himself narrates, makes friends with Shelleys Percy Bysshe and Mary (and Harriet, Shelley's first wife), with Byron and Polidori and with various others. The making of the monster is transferred from Switzerland to Limehouse, a London setting which is nicely evoked (Frankenstein is horrified by conditions in Whitechapel: 'it is monstrous! And it will create monsters. Have you ever seen such squalor?', 41), and the whole tale is moderately diverting, if dilute. But, to speak comparatively, the novel is not a patch upon (not a patch of skin sewn onto the revivified corpse of) Aldiss's Frankenstein Unbound (1973), which was entirely cleverer, more sublime, more erotic and more satisfying.
The spoiler review: the book's twist ending (viz. there is no monster, there is only Frankenstein's schizophrenic alter-ego wrestling with desires his rational scientific mind can't deal with) is pretty weak, and pretty obvious from about half-way in. As with most twist endings it leaves a rather 'meh' feeling in the reader's mind; although in this case the plot is hardly the point. Acutally Ackroyd's subject is male desire and the male body. The monster is buff-esque, in a monstrous way, ('his was the most beautiful corpse I had ever seen...') and, on being animated, his first action is to try and wank himself off.
You think I'm kidding, I suppose. Here: 'He noticed his penis, still erect, and with a groan he began to stimulate himself in front of me. I looked on in absolute astonishment as he laboured to produce the seminal fluid. What monstrous issue might emerge from one who had died and been reborn? His most devoted efforts were unavailable, however.' 
I looked on in absolute astonishment, indeed. Don't remember that from the original novel.