Let's get to the point! The OSTs are A-M-A-Z-I-N-G.   My review is neither critical nor serious nor in a serious tone, but more like a guide, a persuasion, and an encouragement for you all to watch it. My review is long and is also a copy-paste of my replies to many people here. I won't put spoiler below. There is NO spoiler.
 
1. I did not have much knowledge about the Ancient Chinese drama theme (the names, the huge casts, confusing who is who, the rankings, the imperial titles, the culture, etc.) before viewing this show. Hence, that's why the drama might seem boring coz it's a new thing for me. It's a turn off when you get confused about who is who.  This sounds like a con, but the drama's directors can't help but doing things this way. Also, you can memorize names or get a pen and paper to write down names for tracking who is who.  Although this is a con, there are a lot of pros in this drama to override this.

2. This show is the kind of shows that is made for REWATCHING. The rewatching
value is over 9000. Most of the complainers, who said they couldn't get hooked
before the 20th Episode mark or below, needs to know that after they get hooked
and watch all the episodes till the end, they will come back to rewatch; AND
rewatching is even more juicier than the first time viewing.  This is why I say this
show is made for rewatching. It's just not made for the A+ first time viewing
experience because most impatient viewers (I am very impatient too sometimes)
complain "it's going nowhere for them, no destination, no goal", "it's boring to them",
etc.

3. This show is the kind of shows that you won't get hooked no matter what
(exaggeration but it could happen to some people). You need to be patient and not
lazy.  I was hooked after EP 14.  

Such long series that is over 50+ episodes like this, it is not condensed.
During my initial viewing I was lazy, so if you're lazy, it's over for you.
You'll find it's boring if you're used to watching 16-20 episodes Korean or other
asian romance dramas. IF you are into romance, then it's even more game-over for
you. In that case, I'd recommend Ten Miles Of Peach Blossoms (TMOPB) if romance
 is your preferred genre.

Coincidentally, NIF is just like TMOPB for me, I was hooked after the EP 14th mark.
Long series that have a lot of casts need time to introduce the characters.

MY ADVICES:

- Just bear the beginning episodes below the 20th episode (at most). You might even get hooked below that, and try to relax when watching it, don't think to much about who is who, let it flow, then complete it, and now it's your second time viewing. The second viewing will answer all the Qs.

MY VIEWING JOURNEY:

I had watched 12 episodes of Nirvana In Fire, dropped it (nah, more like put it on hold for 10 months LOL).  After that I completed Ten Miles Of Peach Blossoms (TMOPB), took a month break (watching other dramas), and then back to NIF (from the 12th to the 54th episode marathon). Luckily I still remembered most of the things happening before EP 12 so I was set.   I was too lazy to start it from the bottom (1st EP) so I started from where I left- the 12th EP.
I was like, "f**k it! I wanna finish it for the sake of finishing it." Have you ever had that urge? I wanted to complete it ASAP coz I had started it already 10 months ago/almost a year ago. It's shameless if I can never finish it.

Now I'm rewatching NIF from the 1st-54th episode. I encourage you to watch this ASAP but leave a free time for this coz you might need to marathon. Once you start it, you won't stop. hahaha  I am so biased. The only thing I rated a 9.5 is its story. The rest are 10/10. Too afraid to be too biased so NIF can look more legit.