One of my favorite Investigators, if a bit of a sleeper compared to other Mystic investigators.
While Jim Culver has the basics of a good Mystic Investigator, he does not have the longevity of his spells like Akachi or the Sheer Willpower and low health enemy removal as Agnes. Nor does he have the potential of controlling Doom and using it to his benefit like Marie Lambeau. However, how he plays is something more subtle, with the ability to use the Skull token at a constant "0" value in most scenarios. This ability it still pretty powerful for balancing the Chaos Bag in your favor. (Note though you will still have to deal with the effects of what the skull token brings for scenario and even on player cards).
Being a Dunwich investigator as well, he has access up to five out-of-class level 0 cards. This means you have a bit of flexibility of building out Jim how you see fit. This makes him very good in terms of playing him as a strong solo investigator but also an investigator that can slot in well in multiplayer as a support character. And one other benefit is that he has one more health compared to Agnes and Akachi, which does make him more survivable.
A downside to the jazz player is his low agility and having one less willpower compared to Agnes and Akachi. However this is made up with Mystics having ways to buff their already amazing willpower and spells that cover their stats. And while his Intellect and Fight is at three, he is still going to struggle with hardier enemies and high shroud locations without the spells.
If you are looking for an investigator who is provides an interesting challenge to deckbuilding but has a large degree of both reliability and versatility, Jim Culver is your main man.