I picked this up due to its appearance on this io9 list of the "Top 10 Greatest Science Fiction Detective Novels Of All Time". And it's not bad.
The title refers to Mack Megaton; he is seven feet tall, around 700 pounds, and a nearly-indestructible robot. Originally designed by a mad scientist to lead a robotic horde for purposes of world domination, he went straight; as the book opens, he's a cab driver in Empire City, a megalopolis of scientific innovation (mostly dangerous) and random mutation (thanks to all sorts of scientific innovations gone awry). Mack is also the recipient of the "glitch", a poorly understood software bug that's provided him with free will.
By sheerest coincidence, Mack lives next door to a family with two mutant kids. Things kick off when they are inexplicably abducted, and Mack is targeted by the perpetrators for deactivation. He's plunged into the middle of a vast conspiracy, where nobody, robot or biological, can be trusted. Along the way he meets up with all sorts of (mostly literally) colorful characters, is in constant peril, and engages in major and minor episodes of explosive mayhem. And as the book develops, he masters the most important tool in the hard-boiled private dick's arsenal: the well-timed wisecrack.
It's a lot of fun, although I'm not sure I'd put it on a ten-best list. As the plot develops, Mack relies a lot less on his wits, and more on his bulk and weaponry. That can get old after a couple dozen pages. But if you're a fan of old-timey private eyes and gadgety science fiction, there's a pretty good chance you'll enjoy it.