Typical Korean anime-style RPG: heavy on grinding, light on practically everything else.
It's hard to write a catch-all review considering the diversity of players and playing styles, so I'll just have to go with the cliches. Basically Flyff is a cute grindfest, eastern (specifically Korean) MMO-style. If you're looking for lots of non-combat skill, story, and (non-grind) questing, you're looking in the wrong place. Much like RO, there are several cliche classes, starting from the generic vagrant class which everyone joins from lv1-lv15, then the "first level" classes, which branch out into two at level 60 ("second level" classes) for each class.
The stats are, as expected, pitifully few: Strength, Stamina, Dexterity, and Intelligence. Needless to say, the diversity of character builds isn't large ("zomg dex ranger si teh p0w"), so if you're going to play effectively you'll end up with one of the cookie-cutter designs. This isn't helped by the traditional level-up-and-get-this style skill system. Worse of all, the english version of Flyff (aka eFlyff) is licensed from the Korean one (aka KFlyff), and the people running eFlyff (Gala-Net Inc.) apparently have no code access or are prohibited from changing it. Thus, the numerous bugs which infest the game remain unfixed to this day, all the way to version 7 of the game (current as of June 2007), some of which break the game (e.g. the random crash when opening Party or Guild window bug) or severely impact entire classes (e.g. the bugged Elementor final skills).
Technical doom-and-gloom aside, Flyff is fairly nice to look at and play - it runs pretty smoothly even on laptops (I play mostly on my Dell Inspiron 6400). The music is okay, but fairly generic - a problem with most of the studio's (soundTemp) songs. You can play in windowed mode too, which eases up alt-tab paging between the game and your other stuff, e.g. IMs or browsers.
The concept of partying is also slightly different in Flyff; it's just not possible for one healer-type to cope with more than one partner, unless he's much higher level and just there to help instead of in the party for exp. Most classes have builds which basically raise Stamina in order to survive close range contact with monsters, mages included. Yep, you can't go all-Int like in RO for example - unless you like dying a lot, or have a personal heal slave.
Also, parties have a cap of 8, and when full, grant extra exp for the members. Party members must be near at least one other member for the party to gain exp - party exp grants party levels which unlock party skills, so yeah it's a good idea to be partied. Being close to more than one other party member may affect your exp gain though, especially if your level differences are large, so most parties consists of pairs together doing their own thing. So a full party may conceivably consist of 2 high level guys (lv100+), 4 random people who are often referred to as "fillers" (i.e. to fill up the party slots in order to achieve the full party bonus), and 2 low level guys. The pairs will contribute to party exp, the solo fillers basically just benefit from the full party bonus. You could have a party of 8 soloers, but in this case nobody is contributing to party exp, so it's not really that good. The party leader especially benefits from being paired as he's the one who can use the party skills.
Aside from that, Flyff also suffers from the usual low item variety ("at level 100, every archer uses Godly Bow Of The Heavens"), except for the small saving grace that there exist set items ("green"), like in Diablo 2, where wearing more parts or a complete set will grant extra bonuses, and that all items (included the set ones) may drop from monsters with bonus stats (e.g. Helm of Dork, +2 Strength). Also, smithing your items to give them bonuses (e.g. Helm of Dork, +1) will lead to extra bonuses if all your armor pieces (4 = helm, suit, gloves, shoes) are all with the same bonus, e.g. +3, +4, etc. until +10. So, a majority of the more long term players usually spend their time looking for statted set items, and try to smith them to high levels. Other than that, item progression is mostly linear - e.g. bronze sword is beat by iron sword, which is beat by steel sword, then by mithril sword - note I'm not using real game items here, just showing examples, so don't go ingame asking about mithril swords lol.
Sure, there are severel unique items which offer some sort of extra bonus other than stats, e.g. a Wand which decreases casting time, but that bonus (i.e. the decrease in cast time) is not a random bonus which can occur on any item like the stat bonuses, it's just specific to that particular weapon. So again, Flyff mainly consists of generic items. Really. All the high level players will eventually end up with their preferred "best" set items, possibly enchanted to +10 if they can afford it. Almost no exception.
Aside: I don't know why game designers always FAIL at item variety. Set items or unique items should NOT be the top tier equipment; that results in everyone wearing them eventually, as I mentioned. They should be second tier, at best. The best items should be randoms with good stats AND other bonuses. I don't see how they are worried about possible game balance, since most bonuses are stupidly low (the highest a "slot" card item can grant, for example, is +7%, and needless to say it's not easy to find that, and you can slot a maximum of 4 on a suit, ONLY (no slotting helms, etc). A total of, say, 4x +7% hp is a mere +28% hp boost; that's not gonna make you godly. Look at Fate, for example; I love that game: no stupid +1 hp bonuses there, they're all like +10% at least, and it's common to have like +30% mods. It's easy to compensate, just introduce some variety in monster stats, duh. As it is, you can almost write a formula to decide at what level a certain character in a certain class with certain stats is good at tackling certain monsters. That's just plain boring, no variety at all.
Overall, there isn't much to recommend to the game other than it looks fairly decent. There is little depth to it; most, if not all, of the NPC quests revolve around killing monsters for specific drops (the other few consist of finding some other NPC), and the main thing which keeps people playing is the hope of amassing a complete set of stated set items. There is nothing to do outside of combat, other than vending items and chatting. It's a pure grindfest at it's core, with a somewhat funky party system that's more geared towards pairs for optimal exp gain, and where getting up close is practically inevitable - even range classes like the mage and acrobat have Stamina in most of their common builds, in order to survive.
I suppose I'm giving it fairly good marks because I'm playing with real life friends, and you know how playing with good friends always boosts the experience you get from playing a game, any game. Playing by yourself, you may find Flyff quickly boring and shallow. If you're looking for a deeper role playing experience, look elsewhere. As a timewaster though, Flyff fits the bill decently.