Nana Mix! is one of those titles that fully live up to ones expectations: I expected something mediocre and I got exactly that. Reading synopsis is enough for reader to know what to expect from this story. It's simple, predictable, rushed and forgettable. Despite having "20 to 1" female to male ratio and "harem" genre it didn't feel like harem at all. Comedy was really weak. The scenes that (I think) were supposed to be serious turned out cheap and cheesy. There was no tension at all, which can also be seen as a positive - there was no real drama in this manga. Most ofit was slice of life with typical rom-com moments and fanservice.
Thing that kind of saves it is the conclusive ending done in a way that I really liked. Seriously, many better titles should've learned from Nana Mix! how to make nice epilogue chapter.

Characters managed to keep up with the level of story, that is bland and uninteresting. They had pretty much no backstory, no real development and weren't cute or funny enough to carry this manga. From fanservice point of view there was not enough variety.

Art was just average. It wasn't good but there isn't much I can complain about. Sometimes things were hard to read because of effects and backgrounds were weak, but that's it.

Overall I'd say Nana Mix! is ok manga. Despite bad story and characters it was enjoyable enough to finish and ending left me satisfied. Being only 18 chapters long there is no reason to don't give it a try.