The game is fun and very addictive, specially if you're into PvP. The game really shines when you are level 50 and above because you can now feel the diversity of the class selection.
You really have to plan to the very beginning if you don't want to mess your character set-up.
The game is grindy ( give me an MMORPG that is not ) but has many ( yes, MANY ) repeatable quests that rewards you not only with exp but also with items such as potions.
The game can be played solo with every class you decide to make but if you want a rapid increase in level, try to invite some friends and party. Me and my friends reached level 55 at around 2 weeks.
If you're a fan of hardcore MMORPG that has a lighthearted theme, Luna Online is a very safe candidate.