It features a highly ornamented soprano line which carries out the chorale tune. The pedal line moves in constant eighth-notes and the two middle voices (alto and tenor) are given to the left hand.
If you watch the performance of the great Ton Koopman on the Silbermann organ in Freiberg, you may feel you are in heaven for these 4.5 minutes (at least I do).
I think one of the reasons it is so charmingly beautiful is the way Bach ornaments the melody AND at the same time let's us experience this fantastic harmony, chord progressions, and modulations.
This composition, by the way, is a superb piece to play for your church service, public recital or just for the enjoyment of your friends or family. The difficulty, though, is in reading Alto C clef from the Bach Gesellschaft Edition.
Also, since every voice is so independent here, you have to be able to listen to each part separately and analyze the form, tonal plan, and cadences in this piece. Otherwise, you will not be able to appreciate Bach's spectacular part-writing, voice-leading, harmony, and modulations.
If you want to master this beautiful piece, check out my brand new BWV 659 Home Study Course in which I will teach you everything that you need to know to be able to play it in public. The score with the complete fingering and pedaling using treble and bass clefs is included with this course for easy practicing.