I know Astolphe de Custine mostly as an esthete, failed novelist and minor figure (and pretty much a social outcast, being an old homosexual and sugar daddy with the wrong political opinions for the more important Salons) gravitating around Frédéric Chopin (and also Balzac, Hugo, Lamartine, Delacroix, etc.) in the 1830s (George Sand and most of the Romantics like the Liszt-d'Agoult couple hated or loathed him, so Chopin and him grew distant for a while). I've mostly read (and help a friend translate) part of his correspondence, some reviews etc. though I have read many excerpts from his travel diaries along the way. Don't buy it expecting a work of the importance or quality of Tocqueville's, though the themes/topics forcibly brings comparisons. There are good reasons why his works have been totally forgotten for over a 100 years, before La Russie en 1839 attracted interest again. Historians got interested again mostly because his commentaries on Nicolas II's Russia often still applied to post-war Soviet Russia. For the rest, he interests mostly Gender Theory scholars.

From his correspondence, I wouldn't describe him as a great intellect (I've read several letters to Metternich, all of them very boring, so were his comments on Napoléon or Monarchie de Juillet) - Sand is mostly right about him I think, though as a disciple of Abbé Lamennais and cohorts she was deeply biased against Custine's politics, and despite his criticism of Nicolas II, she found him much too kind with the Czarist regime). From what I've read from him, his style in laborious and approximative, not quite comparable to the good writers of his time. His writings on England (though they long predate his book on Russia from the 1840s) were a long series of clichés.

I would say that on the whole he's the not so brilliant son of a more interesting mother, and despite her lover's (the great diplomat and writer Châteaubriand) best efforts to make something out of him, he's never amounted to much.

But maybe his book on Russia is still worth it for all that.