Nana Mix! is a standard ecchi, harem type Manga. Filled with rather Superb artwork, and the individual characters themselves are constructed pretty well. I, personally, liked the manga as a whole. However, there are some drawbacks that are blatantly obvious once the reader is done, assuming one gets that far.
The first, is that the second half of the manga just feels...rushed. As though Akane Kotetsu was told that the manga was being shunted out of existence, and she tried to at least wrap up the story arc before it's inevitable demise. while the last chapter was you regular semi-open-ending, the rest of the second half just made no sense. The Second, is the lopsided character development....in the story itself. For example, you'll learn LOADS about nanami, the female lead in the story, but little if not NOTHING about the Kotaru the male lead in the story. You'll learn only what is needed for the given situation -which may consist of a single event- and then that'll be it. There's no Exploration of Kotaru's past, his mannerism, or his personality. Same with other characters, such as Yoko and Kira. Not to mention Nanami's Gandfather. In story Character development is seemingly focused on Nanami. Perhaps if the above scenario (the rushed ending to wrap it up) is in fact truth, then we may have seen more character building had the manga lasted longer than just 18 chapters, see more of what makes Kotaru Youko and Kira -especially Kira- tick. Third is the extreme subtlety of the romance. One can feel the prolonged plans of the Manga when they realize that...the romantic progress is slow...painstakingly slow. Yet as mentioned before, if the above mentioned scenario is truth, any possible romance is left to the semi-open ending and up to fanfiction writers to complete.
As for pluses...I am Tad Bit biased. I Like to laugh, and Romantic Comedies with very good art always makes me enjoy the a series. I personally like the Comedy the in the Nana Mix. Which ranks it high in my book. It has a lot of fanfiction material, which makes it susceptible to both good and bad writers. So, overall the manga is average , in my honest opinion, but certain things make it interesting enough for me to continue reading till the end.