Beneath the twisted characters, weird, unrealistic surgeries and bizarre, over-the-top situations, there is quite a bit of cleverness that has gone into the writing. People who prefer standard, tried-and-true or realistic plot-lines and those with a dislike of cynical humour will definitely hate this manga.
The plot is quite original, but very episodic. If you are the kind of person who expects every single character to have an angsty past and a romantic interest to rescue them from their 'inner darkness', you should just leave right now.
In writing Franken Fran, Kigitsu Katsuhisa intends to depict the true nature of the human mind and explore the lengths to which people go when trying to achieve their goals. That being said - the characters that Fran operates on are very extreme examples of certain mental illnesses and negative personality types...but that is where the black humour lies.
People claim that this is guro, but I strongly disagree. There are definitely no attempts here to make the bloodshed and violence seem erotic or pleasurable. All gory scenes are depicted in a strictly matter-of-fact fashion; and while there is a fair amount of nudity (after all, most operations can't be performed while the patients have their clothes on!), it is by no means brainless ecchi.
As for LUEser's claims on the manga's repetitiveness - as I mentioned before; Franken Fran is an episodic story that follows different people under different circumstances, with Fran and her surgeries being the only thing that binds them together. The categories section is here to help people; so if you can't do without a continuous plot-line, don't start reading anything with the 'episodic' tag!
All in all - quite an interesting series; a nice, refreshing change from the typical, forgettable nonsense that reigns supreme in the shounen demographic.
Edit: The ending, however, leaves much to be desired. In fact, the final chapter reads just like all of the preceding ones; but even an episodic series needs something...more for its finale in order to be wholly satisfactory -- something Kigitsu Katsuhisa failed to deliver. I'm not quite sure why the rushed ending was necessary, seeing as how the series was reasonably popular in Japan, but while the conclusion was disappointing, my previous opinions still stand.