Bob

First off, kudos to all the wonderfully written reviews before mine. Honestly...well done, I enjoyed and agree with everything everyone wrote.
I just want to add a few things. For me the biggest issue was the casting of Wen Guo. I don't know how anyone could ever think that this actor could play a 24 year old? He was 38 or 39 at the time of filming and he LOOKS 38 or 39. And the shows early years when everything important happens, is way more impactful and takes up more screentime than the series' later years...so it was ludicrous to think that they needed an older actor to "play" younger. It should have been the reverse. If they can age-up the young actress playing Yong Hong effectively, they could have done the same thing with an actor who looked 24. The actor playing Wen Guo was undeniably good, but his characters greatest scenes were the ones when he was younger, yet he was never convincing. Very poor casting decision.
It also bothered me - unless I missed it!? - that we never know why Xiang Dong wasn't with the Crime Division in the City and was in the mountains working at the Detention Center. Seriously...was there ever exact information as to what happened to him? If I missed it, my apologies, but I kept waiting for a flashback or specific reason why he wasn't with that division any longer. Seems to me an over-sight. If he was "sent there" as implied, tell us why!
Also, how did Wen Guo get out of the Frozen Truck? Poor storytelling. We see him later alive, so he didn't die in there, but it's crappy not to give us a 30 second scene that explains it. Especially since they made such a big deal of him fighting for his life and almost getting caught. It's a cheat.
Also, what did Wen Guo do for a living the 18 years or so after he survived running back into the mine shaft and not getting killed, whereas Xi Ma the cop did? (Hmmmm, to that!) Like someone else mentioned, it might have been nice to understand through flashbacks how he got to where he is after all those years. I still don't know what his exact job is/was once he returned. But he seemed to have a lot of money now!
Also, they didn't really handle the whole death of Fenfang as well as they think they did. It really was bit confusing and if I'm correct, the way they played it out at the end, it seemed that for all 13 - 18 (?) years after she died we're supposed to believe he was delusional to her death? But once he "solves the case" he's suddenly aware she's dead and chooses to kill himself? Hmmm.
And what did he do for all those years before Wen Guo returned? Was he at the detention center all those years? Was he driving a mini-car Taxi? Hmmm. S
The acting was great, the humour was nice, and the cinematography was incredible. Some scenes were emotionally true, others were a little maudilin but not to detract from the series overall. It's a shame though the "mystery" was so easy to figure out. As someone else mentioned correctly, it's not really a crime drama , with all that that implies for an audience, but it was different and I guess that's okay.
But it'd be nice if this could be remade by another country in the future. I'd love to see if they can keep the slice of life stuff, but make the mystery a real mystery, actually hard to figure out until the very end.