Firstly, it gets a 9 from Togashi being more than a little lazy and stopping the manga which put a LOT of holds on the anime, which is why the series was cancelled and OVAs were put out instead. The only problem there is that if you watch the whole series and the 3 OVAs, Genei Ryodan saga and Greed Island and Greed Island Final, it will leave you wanting more which you can't get without reading the manga(which currently isn't that high in US releases). Essentially it is a story of four friends (a la Yu Yu Hakusho which is another Togashi creation). The central character, Gon, grows to ~12 without knowing his father. Meeting a man, a Hunter, who claims to have known his father sets Gon off on his journey. He meets the 3 others in the exams to become a Hunter and they remain close, despite physical distance at times.
The story covers all 4 and their reasons for becoming "Hunters". Gon wants to find his father; Leorio, slightly older, wants to become a doctor and help people; Curapika(spelling/pronunciation varies for his name) wants revenge; Killua, Gon's closest friend of the 3, was bored. They all have reasons from their past. It also follows them after the Hunter exams are over, which is when the story really picks up, as does the action. The world suddenly gets a lot larger and more characters are introduced, both helpful and as hindrances to their goals.
It is well-written but with filler, as most anime interpretations on manga do. If you enjoyed Yu Yu Hakusho and other 'fight-based' animes then this is something you'll want to see. Also, despite the ages of the central and main character (Gon and Killua, respectively), it gets pretty dark and sometimes outright violent. If it's an appreciation for 'supernatural' powers such as ki and other odd things, then this is also for you as it becomes an increasingly important issue as the series moves forward. At first it remains fairly realistic and gets odder and more alien as it progresses with the four learning how to use their "Nen" which is their worlds version of Ki. There are 6 types of powers which all effect the type of things people can do with their power. Reinforcement is enhancing physical abilities such as strength, speed, and even reinforcing objects to make them stronger; Transformation is changing the properties of one's aura, such as making it sticky like gum, bouncy like rubber, or carry properties such as electricity; Emission is the ability to shoot aura and control it from a distance; Manipulation allows the user to manipulate objects/people in different ways, depending on the user; Materialization lets the person create objects with their aura, being closely tied to transformation; Specialization is the final one. It is just that, special. It is very different depending upon the person, as on people can steal other abilities, another can mimic the other 5 types, etc.
Hunter X Hunter moves in a gradual process which leads you to the shift from more toned-down fighting and story into the powerful and expansive world that it becomes without throwing you in with no reason. That's another Togashi move, if you've ever seen Yu Yu Hakusho.
