Niecks wrote: "Is there not something pleonastic in the title "Fantaisie-Impromptu?" Whether the reader may think so or not, he will agree with me that the fourth impromptu (in C sharp minor), Op. 66, is the most valuable of the compositions published by Fontana; indeed, it has become one of the favourites of the pianoforte-playing world. Spontaneity of emotional expression and effective treatment of the pianoforte distinguish the Fantaisie- Impromptu. In the first section we have the restless, surging, gushing semiquavers, carrying along with them a passionate, urging melody, and the simultaneous waving triplet accompaniment; in the second section, where the motion of the accompaniment is on the whole preserved, the sonorous, expressive cantilena in D flat major; the third section repeats the first, which it supplements with a coda containing a reminiscence of the cantilena of the second section, which calms the agitation of the semiquavers. According to Fontana, Chopin composed this piece about 1834. Why did he keep it in his portfolio? I suspect he missed in it, more especially in the middle section, that degree of distinction and perfection of detail which alone satisfied his fastidious taste. "

James Huneker: "The Fantaisie-Impromptu in C sharp minor, op. 66, was published by Fontana in 1855, and is one of the few posthumous works of Chopin worthy of consideration. It was composed about 1834. A true Impromptu, but the title of Fantaisie given by Fontana is superfluous. The piece presents difficulties, chiefly rhythmical. Its involuted first phrases suggest the Bellini-an fioriture so dear to Chopin, but the D flat part is without nobility. Here is the same kind of saccharine melody that makes mawkish the trio in the "Marche Funebre." There seems no danger that this Fantaisie- Impromptu will suffer from neglect, for it is the joy of the piano student, who turns its presto into a slow, blurred mess of badly related rhythms, and its slower movement into a long drawn sentimental agony; but in the hands of a master the C sharp minor Impromptu is charming, though not of great depth."