I hadn't read Huckleberry Finn in over 30 years, so I was a bit concerned if I could follow James without re-reading that first. I needn't have worried, that was no problem. The plot is a bit jerky ('and then something unexpectedly happened out of the blue'), and that clearly came from following Mark Twains's plot, but aside from that it is a real page-turner.
Of course, the big difference is that it is written from the point of view of Jim, the runaway slave ('James') and that inevitably means that is all about slavery. Percival Everett vividly and portrays the horrific life of the American slave at the time of the start of the civil war (1860s) and that is what makes the book so special and also so painful to read. I expect that lots of school boards in Texas and Florida will be banning this book, it is full of uncomfortable truths. 
One of the difficulties that I remember having with the original book was its use of dialect, which was not easy to read. I thought that what Everett did with that was very clever and not only made it easier for the modern (and not American) reader, but made an interesting point whilst he was doing it.
The book had a special resonance for me as I lived for a couple of years in Illinois, not so far from Hannibal (where the book starts) and spent the majority of my working hours out on the Illinois river (a major tributary of the Mississippi), including when it was in flood. An unexpected bonus was that the protagonist turned out to have detailed knowledge of local mid-Western plants (I worked as a botanist in Illinois).
This book is both a gripping adventure story and a book bringing an important aspect of American history to life.