It seems most reviews for Tale of the Nine Tailed are either praising it as a masterpiece, or saying it was "ruined" by the ending.
The truth is somewhere in between.

What we do get is an overall decent series, with too much dull filler and too many plot holes.

I think the creators were aiming for something along the lines of A Korean Odyssey (blending the Drama, Supernatural, Comedy and Romance) but whereas AKO kept me consistently engaged, I was snoring through far too much of TOTNT.
To me it seems like the concept was greenlit and rush-produced before there was time to flesh out the background fully, and the show unfortunately didn't live up to its potential because of it.

I think, overall, casting was good..
Dong Wook Lee (playing ML Yeon, a Gumiho in humanoid form) is always solid, but the real standout of TOTNT is Bum Kim (Rang, Yeon's half-human brother), who steals every scene he's in.
Some people here didn't really like the casting of Bo Ah Jo (as FL Ji Ah, producer of a Television program on Urban Legends), but I thought she turned in a solid performance - I think the problem here may be that the character of Ji Ah isn't written as particularly likeable, and - as another reviewer pointed out - is a bit of a Mary-Sue.
Jung Nan Kim was great in her commitment to a role that I think was written as too cartoonish.
Special mention to Hee Hwang and Yong Ji Kim, whose characters could have ended up creepy and hateable respectively, if played by somebody else.
The rest of the cast were fine, but not standouts.


The Bad:
-FAR too many boring scenes of Yeon and Ji Ah strolling down the street together, looking into each others eyes etc. It's like they decided a couple of episodes in to turn this Supernatural mystery into a sappy romance?
-FAR too many boring past-life flashback scenes. Most of these were essentially the same as above (sappy romance) and added nothing to the show
-SO. MANY. PLOTHOLES. I won't list spoilers here, but there are so many thanks that, on reflection, don't make any sense.
-There are other parts of the story that don't necessarily not make sense, but should have had some further explanation. (like finding out that Ji Ah's parents traveled to a mysterious island years ago - but no explanation as to why???)
-The creepy, almost Horror-like vibe we open with in the first couple of episodes just disappears - it would have been good if this had continued through the series
-Under-utilisation of the Supernatural/Fantasy elements - we could have seen Yeon and Ji Ah encounter and deal with different different mythological beings through the series, but instead are almost immediately introduced to the big arch-nemesis, and the rest of the series is focused just on him
-Almost all of the Gatekeeper stuff felt cartoonish and ridiculous, and was only saved by Jung Nan Kim's performance

The Good:
-Bum Kim as Rang was just fantastic, I have to say it again
-Shin Joo and Yoo Ri's romance was cute and quirky and good to watch (AS OPPOSED TO THE BORING YEON-JI AH ROMANCE)
-The mysteries the show presents are pretty interesting, even if some of your questions are left unanswered after the series ends
-The action scenes are pretty good, though they wane after the first couple of episodes

Overall:
I think it was worth watching, despite its flaws, if you're into Fantasy/Supernatural-tinged K-Dramas.