I have tried a couple (and seen my cousins play a few others). I willl say out of all of them that I have played or seen, I liked Rappelz the best.
Is it perfect? Far from it. But of the choices I know of, it's decent.
I played it for a year (average an hour a day) but currently have not played it for a year. I went through my phases of playing it casually through playing it like an addict when I was home for extended periods of time (sick, etc). I was also able to play solo or in a group fairly easily (albeit some classes are better at being solo if they want while other classes have an easier time finding a group if they want).
The graphics maintain a "style" that I like (it doesn't look cartoonish, like Maplestory). And it has the whole sword/sorcery theme which is what I am drawn to (and doesn't have items like guns or technology floating in the mix).
At the higher levels it has little to offer, so I made new characters of different classes to mix it up. If you have all your characters on the same account and on the same server you can trade items between them (of course, not all items are usable by all classes).
The game continues to be developed so every few months new things are added.
It's best, however, if you find a friend or two that also plays because some of the community members are very lacking in the maturity department. But that is to be expected in a free MMORPG where anyone with a computer can play (including the 12 year olds without a job or sense of social decency). I also get annoyed when others become demanding on how I should play or what skills I should have invested it... but you have that issue in any MMORPG (free or not).
Since it's been a year since I've played it, I have no idea what the server load/lag issues are like these days. But it's very easy to pick up the game and controls.
But, all in all, as said, Rappelz is one of the more decent ones that I have experience with. If you have the time and desire to try a free MMORPG, give it a try - free to download and install, free to play. So all it costs is a little time for your effort at testing it to see if it's right for you.
(note: i have no experience with guildwars, as tharkun mentioned, so I can't compare the two, but I have also heard good things about that).
