Brief: I enjoyed this one, but it's slow and quite predictable. That doesn't make it bad at all, but 16 more tightly produced episodes would have been better I think. Cha Shi Ah and Tae Oh however, I thought were brilliant :)

Detailed:
Picked this one up because of Jun Ji Hyun whom I first watched in You Who came from the Stars (by the same writer) and wanted to see her in something different. The mermaid tale is an interesting topic that i've not seen in a KDrama before so that made it more interesting too, and then seeing it had Shin Hye Sun - well, sold.

On the whole I enjoyed it and I didn't have any doubts about continuing to watch it to completion - but - I had some issues with it...

I'm still trying to work out how well the two leads chemistry worked... some times there were sparks, other times not, it just wasn't consistent.

I really liked the links back and forth between the past lives and the current ones, it provided some interesting insights and background and a variety of settings to see the characters in, both in terms of the parallels and differences.

I really wished that the FL had some kind of other goal in mind, but as it was, she comes to the surface and is completely naive and inexperienced which leads to some funny scenes - but sets the scene for the ML to refer to her as his "dummy" the entire story. Now, there are many things that happen in the show that could be seen as just "hey thats fine, it's cute" or not - so there's a lot of interpretation to be had on the "strength" of the FL but i'm gonna go and just say - I don't think she was very strong at all! - constantly chasing around after the guy working out what to do to get the guy - I dunno, didn't feel like the best message. It's a relatively minor itch for me on the overall show (which, as I said, I enjoyed) but an itch never the less.

Now... Shin Hye Sun's character Cha Shi Ah was really good - your understanding of Shi-Ah changes and develops through the story and her oddball relationship with Tae-Oh is brilliant I honestly was looking out for the scenes with these two more than anything else some times.

There's baddies too, in the shape of the "Wicked Stepmother" which I guess has to be there and was ok, but wasn't that exciting for me really.

Sound track was good, really liked some of the tracks that are now ear-worms.

It's a good story but I don't rate it up there with some of the other works by this screenwriter, which include Crash Landing on You and You Who Came from the Stars, two of my favourite dramas. It's just "normal" good :)

The little girl that she meets early on and meets up again with numerous times... wish they'd tied that one up in a nice bow at the end, felt it just needed a moment "more", just as she did with the fashion-lady.

FYI On Viki, this shows up as:
Episode 0 - which is the special, DONT watch this first even though its the first thing they dish up.
Episodes 1-20 - this is the drama, watch this.
Episodes 21 and 22 - these are directors cuts of episodes 1 and 2. Watch these after if you wish :)