TL;DR version: This is an amazing drama. A must watch. One caveat; if you hate gaming or e-sport then give it a miss.
The long version: this drama combined two fully realised dramas into one. What is even more impressive is that it was an ensemble cast of well over ten leading characters. All those characters were three dimensional and acting was great and on point.
With such a complex and interwoven plot there were definitely issues but the show just took you on such a rollercoaster ride that you just hung on tight and enjoyed it.
The promise of the main plot is some kind of temporal anomaly that linked the lives of two crossfire game players, one lived in 2008 while the other in 2020. To help identify the two parallel plots, I'll refer to one as -08 while the other as -20.
First we must address the elephant in the room. If you are concerned about the butterfly effect or time paradox then be assured that the show took a bazooka to them and walked all over their ashes. The show used the time anomaly as a convenient plot device whenever they see fit. All kinds of time shenanigans happened but they all seemed to have minimal impact to the timelines or the world as a whole. Just tell your cognitive functions to go on a paid vacation.
As I mentioned earlier, with such a complex plot(s), there were issues which you can either take umbrage or just shrug your shoulders. The perfect example was the recovery of ML-08 from a prolonged illness. Totally unrealistic but they had to take that route or the show will grind to a halt. Ditto, 2ML-20 which was all nicely set up as a love triangle for OTP-20 and he just disappeared after a couple of episodes. See memo re cognitive functions.
On the whole the 2008 timeline was better written and acted. It just felt more raw and gritty. It started fast and furious and didn't let up until the halfway mark. The 2020 storyline was more like a youth drama with a hint of "The King's Avatar". It could have formed the basis of a standalone c-drama if they wanted. It was well told and well acted but paled in comparison with the 2008 storyline. The 2020 ensemble was still strong but felt a bit cookie cutter (the cosplayer was a standout).  
Around the 20's episodes mark, it did turn very dark and the various plots took a breather while the writer sort things out so that the two timeline can meld into one stream in 2021. A lot of the personal relationships were resolved and cemented around then.
Speaking of relationships, there are three key pairings, one in '08 and two in 2020. They all grew organically and didn't steal the limelight from the main plots. Unsurprisingly, I found the romance of 2OTP-20 was more engaging and funny and they were allowed more latitude with their courtship with several really funny moments while in game. However, having said that, the kiss of the OTP-08 in ep.33 was one of the best c-drama kiss, ever. The penned up longing and passion was overflowing and it was so natural. Swoon.
Speaking of funny moments, it was very clever of the show to use a mix of in-game graphics but more importantly, have the real actors dress up as their game characters and do the fight scenes. There were some tense moments but some of the funniest scenes were done in that alternative game world. Look for ML-08 teaching the FL-08 the game and the bromance scenes later on. Classic, totally classic. I won't be surprised if they recreated some championship games as they were really good matches.
The villain of the piece was also done well. I am a bit tired of the tropey hidden mastermind or Psycho killer with questionable motives. The show's villain suffered a chain of events that got him deeper and deeper into trouble. He was nasty and despicable in the end but you went on that dark ride with him and can empathise.
Kudos also goes to the writer for putting ML-20 in a wheelchair. As a gamer myself, I have been fascinated by the idea that online games are the ultimate leveller when a disabled person can live a full life in a virtual world without hindrance nor discriminations. Ditto giving real gamers cameos and showing photos of past greats in the end credit.
One small gripe is the last 7 mins of the show skipped ahead 5 years but you don't feel anything has changed and there was no obvious character developments. The show hinted is that everyone just cruised along for 5 years which is unrealistic (not even their hairstyle?). After making so much effort to make the timelines feel authentic, those last few minutes just felt out of place. If they have done a 1 year skip and gave us an update on the team/individuals then it would have been a more satisfying conclusion and gave us closures.
OST was good. A mix of battle music, love songs and incidental music. High production value all round.
In conclusion, I suppose I am a bit biased but even if you tone down the gaming side, it is still a good drama. Definitely recommended.