It's quite good by most standards, but also a slog. I had to drag myself through it. It's based on a series of novels that have been adapted into anime, manga and even a 2015 live-action movie. It's also popular in Japan, with the series coming second in Tokyo Anime Awards' Animation of the Year Fan Award, winning the Nogizaka 46 Award (Monthly Newtype Joint Special Award), coming sixth in NHK's Best 100 Ranking Poll and fifth in the female voter's poll.
It's about spies in the early years of World War Two. Japan has a black ops unit called D-Agency, with the slightly unusual motto of "don't kill and don't die". (Their boss thinks death is the worst thing that can happen on an espionage mission, because it attracts attention.) The stories are ingenious, the murky morality is faithfully portrayed and it's all pretty well done... but I didn't care. Firstly, the D-Agents might as well be robots. They don't have personalities, desires or anything else that might detract from the job at hand. If they ever found that they'd started caring about someone, they'd probably quit their jobs in self-disgust. You won't remember any of their names and it wouldn't even occur to you to try to remember which one was which. This makes them brilliant spies, but boring protagonists. The episodes are thus little more than an exercise in plotting, the storytelling equivalent of watching someone wind up clockwork.
There aren't even any story arcs. It's an anthology of one-off episodes, with only occasional two-parters (episodes 1-2 and 8-9). The two-parters are more engaging, I think.
The show's other problem is its amorality. It's hard to get involved in these stories when they're expecting us to cheer for the not-good-guys, which might partly explain why the show went down better in Japan. Admittedly these stories are set before Japan allied properly with Nazi Germany, but even so we're watching the agents of Imperial Japan deceive and outsmart their foes in Britain, America, etc. They send agents to infiltrate the French Resistance. They operate in China, having recently invaded it and committed atrocities (e.g. the rape of Nanking) that the series alludes to at best obliquely.
I'd have liked this far more had the show gleefully gone for villain protagonists. It doesn't. D-Agency are grey. There's just nothing there. They foil their enemies. They laugh at morality and there's certainly no attempt to suggest that they're even remotely nice, but equally they're not villains. They drift through a grey world, being very good at their jobs.
The episodes are... all the same in my head. In fact they're not at all, but my level of engagement was similar with all of them. The show was always last in my "to-watch" queue and I'd have never finished it if I hadn't made a conscious effort to push on. It's the kind of thing you start watching optimistically, but then delete nine months later after realising that you never got past ep.3. Spies do spy things. D-Agency are clever. Their enemies get outwitted. Relative high points include:
(a) inter-agency rivalry. D-Agency has run-ins with Japan's Imperial Army (eps.1-2) and their own rival spy agency (eps.8-9). "Kill without hesitation! Die with honour!"
(b) children enliven ep.6 a bit. That one also has SMERSH on a train, like From Russia with Love.
(c) Nazis are thoroughly nasty, of course. There are some horrific ones at the start of ep.7 and a cool Nazi spymaster in ep.11 who remembers Japan as the enemy from World War One and sees them as unfinished business.
There's a lot to admire in this series. It's wholehearted. It's historically detailed. It's storytelling for adults, making no concessions for likeability or standard otaku appeal. It's committing utterly to its story of grey, bloodless nobodies in a world where good and bad doesn't really matter. You'll have a job to do. There might be some innocents, although they're likely to turn out to be less innocent than they look. However if you're not Japanese, I think you're likely to find it a bit rubbish, really.