There's two ways you can approach this particular comic for an honest, unbiased review.
On one hand: it's certainly got an original storyline and a unique twist to the ending, but on the other hand it's a mediocre (and flawed in some areas) comic.
Firstly: it's designed as a novel. The story is not illustrated like any comic book, with art work plus the narration and the characters' dialogue. The Clown at Midnight is the complete opposite! Text, with some Photoshop/3D art work to accentuate the pages. It may become a nuisance to some readers since the background of the pages is a dark grunge background, so it distracts from the text as well as simply making it hard to read since the small font is black. The cover has absolutely no relevance to this comic regarding art or any illustrations at all.
Secondly: Grant Morrison's writing is hilarious, if not atrocious. Apparently he has a fetish for over-the-top purple prose.'The alarms at Arkham went off like a pack of wolves in heat', anyone?
Thirdly: The penultimate point of the main story is flawed. I cannot describe it here, otherwise it would be an absolutely massive spoiler! You'll know the moment when you read it.
I can't help but feel that at the end, Batman's dialogue and way of words/thinking became very out-of-character, and off-track.
It was a good read if you wish to ignore the run-on sentences, purple prose, an abusive use of similes. His writing for Harley Quinn was brilliant; but if you love good writing skills and an actual 'serious' plot, look elsewhere.