When it appeared, it made quite a splash: the mod's author (whose name is plastered all over the intro and ending titles) took a year off to create the mod, in hopes of getting a job at Bethesda, the creators of Skyrim. And it shows: the island is pretty big and has tons of quests and locations.
There is the main quest, showing the conflict between two families, and there are a ton of side-quests: pretty much every NPC has a side-quest for you. You can easily spend five to ten hours playing all of the content, and that's exactly what I did. By the time I was done, I had cleared all the locations and done all the quests (that I could find...)
It's an impressive mod, no doubt. There's a lot of content, and all the NPCs are fully voiced. (Unfortunately, the NPC whose voice I disliked, had the most lines.) There's lots to see, lots to do.
If I were Bethesda, would I give him a job? I'm not so sure. The conflict between the two families does not allow you to choose sides: there's the Good Guys and there's the Bad Guys, and you can't switch allegiances halfway like you can in the rest of Skyrim. The map is largely 'empty': long stretches without much of anything, with only the occasional wolf or spider. Skyrim seems crowded sometimes, but Falskaar seems empty. The map is clearly divided in three parts, which makes it feel artificial too.
The side quests are fun to do (especially the ones with the necromancers and their dark jewels). And the main quest... it's fine in a way, and there are some really big fights that are interesting because there aren't many of those in the game itself. But the ending is a total let-down, deus-ex-machine style.
So yeah, do play through it, even if it's only to see what the fuss is about. I've had fun playing through it, but there are some spots that are a bit problematic. Also, take note that if you go there, it will take some time before you are able to go back to Skyrim itself!