While Faith wasn't perfect, it still ended up being obsession-inducing. There were so many good things that helped make up for the flaws.
But let's get the flaws out of the way first. The biggest problems were the directing and editing. There were so many scenes that could've been amazing if they had only been executed another way. The music was edited in awkwardly sometimes, too. And the fantasy aspect was underused as well as some characters (Jang Bin, Eum Ja, Hwa Soo, etc). As one of my friends put it "this show doesn't seem to know how good it is."
But let's talk about all the awesome that made up for the flaws. I think the cast was AMAZING. Lee Min Ho did an amazing job as Choi Young. (And I'm only saying that partially out of the fact that I think he's really attractive. ^_^) I think it's my favorite role that I've seen him do. Kim Hee Sun also did an awesome job as Eun Soo. She was funny and quirky and strong without being annoying. And she and LMH ended up having some AWESOME chemistry. The supporting cast was excellent as well. The king and queen, the Woodalchi boys, the baddies, everyone. I think this is one of the few K-dramas that I've watched where I wasn't annoyed with some bad actor.
The storyline was also really good. Not always executed well, but still really unique and interesting. There was a lot of really good character moments and character development. It was unpredictable, funny, touching, action-packed, political, romantic... not always in a well balanced way, but the moments themselves were good.
The music was really good, just not always used well. The costumes and sets were beautiful. So, basically, there's lots to like as long as you can get over the flaws. Faith is one of my new favorites.