Players of Ragnarok Online, you may be able to add 2 or 3 to the rating I have given just because you'll actually have some understanding of what's going on, and you'll find some of the references to the actual game humorous. I myself am a former RO-player, so I actually managed to get some enjoyment out of this series. To the rest of you all who know nothing about the game, I strongly advise you to find another series to look at. There are two main reasons for this.
Number one, the characters. I certainly appreciate and respect the noticeable differences in personalities between the characters, which is usually what you'll find in a typical party in any MMORPG anyway. But the fact that every character, save for one, behaves in a very predictable and stupid manner, and the fact that their behavior remains much the same throughout most of the series becomes an immediate turn-off before one even gets deep into the plot. Which brings us to our next inherent problem...
Plot. Or lack thereof. Which is semi-understandable. The world of Ragnarok Online has no notable "main objective" for the player to carry out either. Much of the game is spent leveling up, hunting monsters, farming items, fighting other guilds, killing other players, etc etc. Naturally, for all of these activities to be translated to a coherent storyline in an anime setting is incredibly difficult, and the series does a respectable job of trying to touch on most of these points. But in the end, the plot is average at best. The driving force(s) behind the character's decision to travel all turn out to be rather lame reasons in the end.
Alright, now I might be more harsh on this series than I should be. To be sure, there are some funny moments here and there, as well as one or two moments when the characters stop being stupid and actually pull off a cool stunt. But these moments are few and far in between for non-RO player viewers, and even for RO-players, they'll have a hard time finding much to like about this series as a whole.