Addicting with a huge side effect of boring.
So far it sounds like your general rpg, so your probably wondering: What separates this game from the rest? Maple is unique, in that, it is a side-scrolling mmorpg (there aren't many of those!) and that the game's mechanics are simple and understandable to almost anyone, even kids.
The game is very addicting because it's so easy to party and chat with friends and meet new people. They're are many builds you can follow for your class, and accomplishing tasks, such as defeating Zakum (one of the bosses in maple story) with a group of friends or guildies, gives one a feeling of relief and great achievement. In the end game it is all about batlling bosses for rare equips that boast the greatest potential and thus increasing your damage beyond imagination. Damage is very important to Maplers and they will spend hours and tons of money to achieve their ideal damage. Some players pay to get married to another Mapler - married people can participate in certain quests such as Amoria PQ.
The graphics are well done for a 2d side scrolling game - nothing is absolutely spectacular or unique about them. The entire experience is 2d - no additive 3d effects. This is good for slow computers. The animations of the skills have been revamped a few times and they are usually smooth and fit in the 2d realm.
Maple Story absolutely requires an utter plethora of time in order to achieve high level status. Many players quit before they even make it to 3rd job (level 70) just because the countless hours spent trying to level get EXTREMELY repetitive and FRUSTRATING. And there is a big chance that some people will steal your map in which you are training. As of 2010, Nexon has changed the experience curve (for the second time) in an effort to make leveling a breeze. While the leveling certainly is much faster - this is mostly for the levels 1-70. Afterwards the leveling becomes slow again - although, it is still a big improvement.
An alternative to grinding is joining a party quest, BUT this is not so easy for you have to have a party of usually 4-6 and only one party can enter per channel! So if your party leader isn't very good, your not going to make it in! As of 2010 the party quests changed for the better. Less members are required and any level can enter a PQ.
The managers of the game, Nexon, do well to update the game monthly and add events during holidays or seasons, HOWEVER, there is a major hacking issue in Maple Story that Nexon cannot seem to control, making training much more difficult and hackers can even diminish any motivation to level up when a person can get to 100 in a few weeks and it takes you several months! As of 2011, there are still many hackers (sad face).
There is a micro-transaction system in Maple Story (cash shop) and those who use it can get ahead in Maple Story very easily. Although you don't have to pay, it's very tempting to go out and buy some nexon cash. As of 2011, the cash shop gives the user a much greater advantage over non-paying members (strictly in terms of damage - both players get about equal game content).
I feel I should mention that Maple Story has a huge fan base/population/community. Not all of them are the nicest of folkes - although, it is safe to say that you will never be lonely. The game is also less likely to die this way as so many of the members use the micro-transaction system.
Overall, maple story is an average mmorpg only worth playing if you have an old computer and love side scrollers. Maple Story is most appealing for children. If you have a lot of free time, patience, and love side scrollers then Maple Story is an option for you. If you're looking for a game that requires some kind of depth and skill to play, Maple is not that game (due to new classes and pvp options - I take back that statement)..