Grimgar, Fantasy of Ash is easily one of the best written light novels-- or at the very least if you like fantasy, you're going to have a blast reading this. But I can almost guarantee you that one way or another, Grimgar WILL leave an impression on you. The characters are well-written, the plot's always unique is some regard(volume to volume) and the setting has a fascinating premise(that I can't say because of spoilers).

It's an 8/10 read, mostly because I'm sensitive to some of the problems that the content had-- most of which may not be so visible to readers who're casually passing the time. It's written by an author who wrote over 80 light novel volumes before starting Grimgar, or something along those lines-- currently sitting at over 100-- you can expect the writing to be fairly satisfactory throughout its entirety.

Characters
Pros
They feel like real people with real thoughts-- one of the author's greatest strengths is his ability to stay consistent within the confounds of who the characters are, without the readers seeing into their minds. You can more or less infer what they're thinking at any point in the series after a instance where we get their point of view. The author continues to simulate and calculate their thought processes in the background allowing for really good moments where they break expectations without it seemingly contrived.

Another interesting detail that I think needs to be addressed is how much respect the author gives to the idea of talent. In terms of base stats, rate of growth, and maximum potential. Many stories make it seem as though anyone can rival a super genius at swordplay, or some other fighting style-- which isn't out of the realm, given the right circumstance-- is simply unrealistic, and a lot of the characters tend to have infinite potential. The sense of realism is strengthened by this.

World
I can only say so much without spoiling really important plot points. Assume that the pay off will be in Volume 5, if it still doesn't interest you, then perhaps the setting simply isn't for you. You can expect a vast world, really cool equipment, techniques, magic abilities that mostly stay within reason-- obviously it does require suspension of disbelief when the best of the best(Akira and Souma) are jumping several meters into the air, but you don't see that from combatants that are less than the strongest.

I want to keep this review within a reasonable length. So I won't get into the plot; you won't be disappointed.
Flaws
I don't like how the author uses romance as a subplot to gain the readers attention but doesn't actually explore it. Considering how much detail and subtext is placed into the general demeanor of the characters, backed by scenes where their inner thoughts are revealed-- there's so much potential squandered when the character's relationships could have been something you wouldn't be able to find in 99% of Romance books(in America or Japan).

Slow pacing is one of Grimgar's strongest strengths because it grounds the character's arcs and abilities-- but the author does a poor job handling plot twists, and does so in a way where the slow pacing worsens it. Consider reading volumes upon volumes of content for characters to progress their relationships, their accomplishments, wealth, and techniques-- only for that to be reset. I'm keeping it vague. A lot of his twists are like that; it's frustrating.

A change of perspective to Souma's side of the story, or Renji would be nice-- but you don't get that here. Occasionally, there's a shift when one of the party members are the focus, but that's about it. Not that big of a deal, but it takes away from the experience.

Conclusion
There's a lot of wasted potential for the characters, but the character writing's still some of the best you'll ever see. Plot and world building is weaved together to make for a fun adventure, traveling to unpredictable places.