i borrowed my bro's DS and played it for a week or less. I am not a big fan of small screen so it's kinda tiresome to play it for a long time.
I started off with a thief, there's a mage and a swordsman that will join your party at the beginning. It's sort of easy (up to where i played, lv 40~), if you don't know japanese at least you know what some of the items does from you RO knowledge (i.e. ygg, red potions etc, equips are a slightly different story). The skills your char get is a bit useless (for my little thief, not sure about other classes). Cards are easy to get. Zeny isn't hard to get and equips aren't expensive.
Let see, I find that they took advantage of many already existing sprites. Such as the chars, the monsters .. of course there are new npcs, and the maps aren't anything like RO online. Wait I think culvert was the same, but that's about it. I heard my friend said you can network play in some tower (near morroc) to increase your level but I never tried that.
I didn't get hooked to it maybe because it's japanese and i don't understand the story. I might try again when I get to setup the English version. Someone gave me the steps before but I am lazy to do it and it's not my DS so yea ...