I have always been curious about the famous Yanxi Palace but never brought myself to watch it due to my comparably low enthusiasm for harem drama and the mountain exercise of 70 episodes.
But after watching the Legends and falling in love with Xu Kai, I became curious about the drama that made him famous overnight.
Seventy episodes and 50 hours later, I can say that this drama was worth my time.

First of all, what I like about this drama:

* Unconventional female lead: I have watched a few palace harem dramas before: the female leads are beautiful, intelligent, kind, soft outside, but strong inside. They manage to charm the emperor early in the show and overcome the dirty tricks played by the evil queen or concubines through their effort and righteous behavior. However, this is not the case of Wei Ying Luo. She has a fierce temperament, fights, lies, and cheats her way to her goal if it's necessary and does not falter before her enemies. It's refreshing to see such a different female lead in a rather typical drama genre.
Some critics say that it's too unrealistic that a character of hers would be able to survive and even succeed in this historical setting. I, on the other hand, likes this drama just because her characters which bring one after another surprise. The show also depicts how she grows from a capricious maid to a respected royal concubine, beautiful character development worth following through.

* Quick payback: what I used to dislike in palace harem dramas is the long and draggy story, at which the villains get their way of doing evil for a long time, and the hero only succeeds after a long time. This is not in the case of Yanxi. Most intrigues and cases are resolved within 1-3 episodes, and it is very satisfactory to watch how the evil characters get punished quickly.

* The perfect fit between great characters and their casts: I am just in awe of all the main and supporting characters. The drama features young but talented actors such as Wu Jin Yan and Xu Kai but also many veterans Nie Yuan or Qin Lan. All roles have distinguished personalities and their own backstories. I rarely felt an action of a character to be unlogical or unsuitable. My personal favorites are the former queen (played by Qin Lan), the Imperial Nobel Consort (acted by Tan Zhou), and Ming Yu (played by Jiang Zi Xin). I hated, loved, cried, and felt sorry for those characters and cannot imagine these roles played by anyone else but the actresses.

* Beautiful romance: I really treasure the romance between Ying Luo and Fu Heng, it's one of the purest and saddest love stories I have seen in dramas. Unlike the romantic emotions Ying Luo showed to the emperor, the feelings she expressed to Fu Heng felt more real. (In terms YL's love towards the emperor, the transition between faking and real love was not clear and logical to me; therefore, it looked a bit fake until the end.) Despite crying buckets in the last episode and feeling heartbroken about my favorite couple, I applaud the bravery of the writer to go down this route as otherwise, the entire story would not have been convincing. Some say that Xu Kai's acting was a bit stiff in his role as Fu Heng. I, on the other hand, don't see this at all. I think he depicted Fu Heng's stoic, protective and loyal personality very well.
I also really enjoy the somewhat different romance between Ying Luo and Qian Long. A hate-love relationship of modern times transferred into a traditional setting is exciting to watch. Nie Yuan did a great job acting out his inner conflict when slowly falling in love with Ying Luo.

* There are many other aspects I also very like about this drama (e.g., the cinematography, the costumes, the OST, the sets, etc.). I would not go into detail as they have been mentioned by others already.

Now a few areas that could have been better in this drama:

* The female lead has unrealistic foresight capabilities: despite knowing that the makers want to create a brilliant female character who can see through many unfair games played on her, the level of foresightedness Ying Luo has is just a bit unrealistic. Logically, there is just no way how she would have known so many things in advance. At the start, the audience is still surprised by her detective cleverness, but after a while, it just becomes a bit silly. Ying Luo should become a psychologist or a fortune teller.

* The last few episodes (i.e., when the kids are grown up) are not necessary, and the ending was unrealistic: For me, the last arc was not required and completely anti-climax. Especially the case on the ship was dreadful. It felt fake, the actions cheap, the resolution unsatisfactory. I understand they want to create a reason to give the queen what she deserves finally, but was it necessary to wait for the kids to grow up? I feel that the makers want to desperately create a final climax that is worth standing at the end of a 70-long successful palace drama, but it went off in the opposite direction.

* Sometimes illogical plot development: I guess it's unavoidable to exaggerate and be creative here and there to build the dramatic. But some get too far. Some honorable mentionings are:
... The court doctor never gets punished despite lying and cheating for so many times.
... The concubines only bear sons. There were some daughters a few years later, but they were just bullet points in the subtitles.
... The annoying and mean-looking concubines are never really evil. The really malicious ones are those who appear to be kindhearted and gentle on the outside.

All in all, I can only recommend everyone to watch this drama. You might want to fast playing with 1.25x some of the dialogues as 70 episodes do feel like forever. But I urge you to watch the entire series without skipping anything as you would not regret it.