Written in the second of Bach's 3 major compositional periods, along with such masterpieces as the violin sonatas and partitas and the Brandenburg concertos, the six suites for solo violincello give a beautifully complex and rich array of music to an instrument with otherwise fairly scarce solo repertoire. Each of the six suites has the same formal structure: a Prelude- usually by far the longest movement, Allemande, Courante, Sarabande - often with complicated double stops, followed by either a minuet, bourree or gavotte, and finally a gigue.
They represent an enormous body of work, taking over three hours to play in one sitting. They also represent a huge and neverending challenge for any cellist. Like so much of Bach's solo works for any instrument, the easier movements can be begun at an early stage, but some movements will be a source of stress for the rest of your life. The sixth suite in D Major, in particular, was written for a violincello piccolo, with five strings, and is a huge challenge to play well on a standard modern cello, with four. The Fifth suite in C Minor is also unusual, with the top A string being tuned down to a G to make playing in such a difficult key signature, full of A flats, a little easier. The fourth, in E Flat Major, is a challenge for its constant and rapid shifts and huge string crossings, particularly in the elegant prelude.
The first three suites, in G major, D minor and C major respectively, are comparatively easy, but can be extremely deceptive. Usually simple in rhythm and bowing, with simple key signatures and a lack of the thumb position work of the big shifts often found in later and ostensibly more difficult pieces, they can be initially learned rather easily, but then prove fiendishly difficult to play very well. The complex, mathematical developments so common in Bach and the constant and irregular shifts between strings mean that playing these pieces up to speed and with good tone quality requires a level of delicacy and balance in both hands that is only found in quite advanced cellists.
A classic example is my own experience with the famous Prelude to Suite 1, in particular the first four bars, which I have been playing for no less than two years, and still struggle to get perfectly even. I was about to give up in despair when my teacher said "Why do you think every recording you'll ever hear of this piece slows down very expressively and artistically in this bar? It's because NOBODY CAN PLAY IT." Absolutely typical Bach.
The Suites form a backbone for standard cello repertoire. You start learning them after about five years at the instrument, and you never stop learning them until you give up or die. They also provide a reliable test of skill - many a fool can play a big, Romantic solo convincingly, but playing a Bach prelude beautifully takes great skill. I believe a similar system is used by many pianists in assessing their peers.
In addition, it is a common experience among cellists that when the suites are mentioned to a random person, any response will be "Yo-Yo Ma!". Let it be said that it is the opinion of many cellists that the Yo-Yo Ma recording is mediocre, lackluster, unadventurous, not particularly skillful, and often, dare we say it, boring. The most interesting and dynamic interpretation I have yet heard is by Dutch cellist Pieter Wispelway, who I was fortunate enough to hear play all six suites live in Sydney two years ago. That performance was what gave me true enthusiasm for this great work.