All in all, one of the best shonen manga out there.
Characters- they are many and diverse, and usually very interesting (and, of course, entertaining). The main cast and certain villains (Genei Ryodan, Chimera Ants and Zaoldyck family, if you want to count them as villains) , as well as supporting characters like the Hunters stand out particularly. Character development among the main cast and occasionally in the Genei Ryodan is fascinating to watch and lifts the characters above being unchanging and boring. The Royal Guards and the King also grow and develop in fascinating ways. Hisoka also gets creepier as he goes, while the Zaoldycks become Affably Evil.
Plot- the fights, while not the most epic in the phsyical sense (they are always excellent, but fights are often physically brief), are epic in every other way. The nen system is well-developed and makes a fight more than just "I must charge my POWERRRR!", and thought is put into how the characters manouver around. Most of Kurapika's conflict with the Genei Ryodan is almost completely non-physical, since after one fight, the only direct attack he uses is kidnapping (after a lengthy game of cat and mouse with each side trying to manipulate the other into a less strategic position), which develops into a hostage situation. Even the training sequences are interesting, but then again I actually enjoy training, provided it gives you new and interesting details about combat in that world.
Also, the conflicts themselves are interesting, such as the fight between the humans and the ants, and between Kurapika and the Troupe.
Setting: The world is quite entertaining, and childish in a sort of warped way.
Greed Island- while alright on its own merits, and with a few standouts (the final fight against the Bombers and the Dodgeball game, and whenever Hisoka shows up), this arc drags. The villains are just plain boring. The plot gets dogged down by elaborating on game mechanics, which unlike earlier details are just not interesting.
Art- usually very good, but there are a few chapters (five or so) that are simply terrible. (These are fixed in the tankoban). There are a few panels that are less fleshed out, but they usually fit in with the author's style so it works, it's just not up to par with his better art in the series.
Overall: Definitely recommended!