Gunsmith Cats is all about guns, muscle cars, and sexy girls. While the story isn't anything particularly amazing, the action is constant, and I had no trouble reading a few hundred pages at a time. New characters are introduced often enough to keep things interesting, but infrequently enough that I could actually keep track of them.
Kenichi Sonada initially made a Riding Bean manga, but after some difficulties with the publisher, he revamped it a bit and made Rally Vincent the focus. This was a good marketing move, as Rally is goddamn sexy (and kickass, like an American girl should be!). The chapter headings show that Sonada was well aware of his male audience - it's not uncommon to see one of the female characters posing with a weapon and less than usual clothing.
The omnibus editions published by Dark Horse are shrink-wrapped and marked for mature audiences. While a bit puzzled by this (having watched the anime first), it became evident later on, as in the first volume you get May having (censored) sex with someone for a few frames, and a full two pages of her masturbating later on that look straight out of an H. Both Rally and May manage to lose their pants once in a fight throughout the series, and nipples will show up when you least expect them, including on a few loli-types. There is, of course, a great deal of violence, although not to the extent of something like Hellsing.
While I am an expert on neither cars nor guns, I can tell that a great deal of attention was given to accuracy in both areas. The portrayal of American Chicago is fantasized a bit, certainly, but doesn't seem to inherit any odd Japanese cultural things, as many series have a habit of doing.
Overall, a fun action-y read on a lazy summer day, and a great deal to purchase.