What's it about?
Setting: Medieval RPG style fantasy land
A group of young people wake up in a strange place with no idea of how they got there, where they came from or who the people around them are. They are given some money and no choice but to form into parties and kill creatures for profit in order to earn the right to become volunteer soldiers. The strongest young man there gathers the strongest looking people and heads out. We follow the party made up of the members left over as they join guilds, learn skills and struggle to master their new powers and weapons just to survive.
How's the fanservice content?
The fanservice is treated quite differently to some other shows in that it is almost entirely tied to the specific male gaze of characters on-screen. Dark knight Ranta, blunt and aggressive, uses sexualised insults and actions to belittle the women around him, while thief Haruhiro finds his eyes and mind sometimes wander to his female party members' bodies before he can stop himself. The camera often follows Haruhiro's eyeline (including when he catches himself and jumps from breasts or legs back up to face)
We do not see the other male characters responding to Ranta's comments (unless you watch the Special episode, which I recommend you don't). Haruhiro is the POV character, and the camera taking on his view as he is attracted to the women around him while remaining determined not to sexualise them feels like a realistic struggle. It means that, a good amount of the time, the fanservice moments are used for characterisation, both for Haruhiro and for Ranta.
The kind of abuse Ranta throws around is playground level insults and pranks, mostly telling Yume she's flat-chested or trying to sneak a peek at the women while they are naked, which leads to Haruhiro following Ranta's train of thought, which leads to fanservice close-ups of the women's bodies. This may make it easier for you to watch, as it is generally incorporated into the story very organically, or it may make it harder to watch as some moments are a little too true to life. The worst scene is in the pilot, when Ranta draws attention to shy Shihoru's larger than average chest, for no reason at all.
The other moment that stands out is Haruhiro's introduction to the pure fanservice character that is his thief guild teacher, Barbara.
Frankly, the pilot has the worst fanservice moments of the entire series. I don't blame anyone who decides not to get past that, but if you do there is some really worthwhile worldbuilding, storytelling and character development in store for you, especially from episode five onwards.
What's good about it?
"Modern day people are transported to RPG style fantasy lands" is hardly an original premise, but it's one of my favourites. The twist here is that none of them remember their previous world, so they walk into this new world with the same level of knowledge and skill: zero. If you enjoyed that aspect of Konosuba, I do recommend watching this to see how it's tackled as a straight action drama.
Haruhiro is an everyman character of a type that is common in anime, but unlike so many everyman characters, whose job is simply to be neutral while interesting things happen around them, Haruhiro himself undergoes the biggest character development over the series. In episode one he is forgetting to do chores, snapping at teammates he clashes with and accidentally checking out party member Shihoru's chest. We see him slowly learn through trial and error what it means to contribute to and strengthen the team without causing unnecessary conflict.
Haruhiro is the opposite in almost every way to Ranta, who has no interest in minimising conflict or treating the people around him like friends. I would have preferred to see more of an arc for Ranta regarding his relationship with the women around him, to reach some sort of respect for the women he had been insulting and embarrassing earlier in the series, but maybe that's being saved for another series. In some ways I appreciate the realism of this; these characters aren't perfect, and while they all grow and develop individually and as a team, they are still far from perfect by the end of the series, individually and as a team. Ranta improves and grows in some ways, but not others.
Execution of its unoriginal premise is where Grimgar of Fantasy and Ash shines. These characters are painfully aware that they need to improve their skills or die, or - in some ways worse - be unable to protect each other in battle. They have limited funds, no friends or family and no choice but to learn competence in the fields they have chosen, regardless of their lack of aptitude in these areas or their aptitude for non-combat skills. It's clear early on that warrior Moguzo, who is the biggest so is given the biggest sword, is much happier making things with his hands than destroying them. Even Ranta, who talks up his bloodthirst, is deeply affected by his first kill even as he recognises the necessity of it.
They have to trust each other with their lives while they barely know each other, and kill whether it feels right or not. Their first kill is significant, and treated with appropriate respect, but is also celebrated as an achievement, a sign that they can actually survive in this world, even though the single kill itself nets them less money than they had hoped. Their achievements at every stage are conflicted, often bittersweet, and the more secure they feel in their survival the harder they have to work to stay close as a group.
By episode five it is clear that Grimgar of Fantasy and Ash is an exploration of teamwork and leadership. What makes a good leader? What makes a strong team? How does a good leader make their team strong? The show asks these questions and works towards answering them while this team goes from dropping weapons and missing their marks to becoming skilled and confident - but still fallible - fighters.
There are certain aspects which are simply left unexplored. The fact that they have come from another world is in the background of the story but quickly becomes irrelevant. The other people who arrived at the same time are shown once or twice, but they barely acknowledge each other. We get a sense that some in this world may have had different personalities or priorities in the world they came from. We don't find out whether every soldier in the town has the same background as them or whether they are a one-off cohort. However, each of these points is ready and waiting to be explored in future seasons, and could make for a very satisfying longer term series.