I'm really startled at all the bad reviews, frankly. I mean, eight people voted 1?! This is not a masterpiece by any means, but it's nowhere near that bad. I actually found it cute and surprisingly satisfying, despite the rather thin plot and characters that needed more development.
The main reason why I'm more lenient than usual with the obvious weakness in pacing and plot is because the first chapter practically screamed "oneshot that was extended due to popularity." It literally reads like it, with the usual semi-vague happy 'conclusion' at the end of the chapter. As such, the best part (of their process of falling in love) was completely rushed - I didn't get a sense of why they fell in love, and the "kissing" theme was clearly a gimmick rather than being an interesting and central part of the story. They're simply kissing maniacs; there's no real reasoning behind it. On its own, the first chapter would've gotten a much lower score.
However, the second and third chapters improves upon the first significantly, and they're the reason that Kiss ni Juuzoku is worth a read. The second chapter expands upon Lilith's feelings and introduces Jean to the thief gang, while the third provides us with further depth to Lilith's character, provides backstory to her crew and wraps up the loose ends. By the end, you understand Lilith so much better than it becomes quite clear why both her gang and Jean is so incredibly loyal to her. I'm still not sure I'm 100% convinced about the reasoning Lilith gave for falling for Jean, but I suppose their actions speak louder than words.
The art is lovely, clean and sharp. If I had to nitpick, I'd point out that there are some instances when the proportions of Jean or Lilith's faces are slightly off, but it's a minor thing. As for the extra story at the end, it was cute, but unremarkable.
Kiss ni Juuzoku had potential, but it's a shame that it seems the author didn't have a chance to develop it as a full length volume from the beginning. Despite doing an admirable job of turning back and building on the characters in the latter chapters, it's never quite the same; there's only so much you can do when the main development at the early stages of their relationship was so rushed. That aside, I still found it a reasonably enjoyable read. For a one volume manga, it's definitely not bad.