It's not often I talk about a manga or Light Novel; partially because I don't like to give reviews on incomplete things most of the time. And most mangas/LNs take ages to finish. However, this is one of those exceptions.
So what is Kanojo ga Flag about? Basically it's about a guy who can see 'flags' on people, which is a reference to dating sims actually. For those unfamiliar with the term, go read The World God only Knows. For those lazy to do that, basically a flag is a Y/N binary state which dictates the flow of events in computer games; especially dating sims. So when you set a flag, you basically lock the flow of events into that certain route.
So this guy can see 'flags' on people, which in the story basically translates to a very loose form of telepathy/clairvoyance. He can tell their basic intentions; if they are friendly/in love etc, or if they have death in their future (otherwise known as the Death Flag). Throw in a bit of romcom, a bit of foreshadowing as the protagonist gains a mysterious Death Flag, a bit of epic fantasy as you gain little bits of insight into why the protag has this power and the machinations of a secret cabal working behind the scenes.
So what's good about this series? Well, the characters are pretty interesting. They're not overly complex and basically hit every major note in the archetypical dating sim genre; ranging from princess, oujo-sama, onee-chan, trap(?), childhood friend etc. However, considering the general self-parodying nature of the manga, you get the feeling that's intentional rather than just trying to be as popular to lonely otaku as possible. However they are amusing, I'll give them that. And that works particularly well when you're writing a light-hearted parody with a dash of epic plot on the side. Even the main character is more than just your average 'hey, imma a kind boy, fall in love with me' kind of wish fulfillment character.
To be very fair, the comedy and light-hearted pace of tone is what drew me in. Unlike most harem series, it rarely tries to sell itself solely based on ecchi(otherwise known as show more skin) but manages to set itself apart by appealing with comedy. Which works very well and is sort of reminiscent of the comedy in Kannagi (another good series).
This started out as a LN, but since I haven't read the LN, I can't comment on them. But based on the manga version; which I have been reading, I'd say this is a pretty solid piece of fiction. It's not some epic fantasy but if you liked Hataraku no Maou-sama, you'll probably like this as well.