Beethoven wrote variations his whole life, from his earliest pieces on, and not just as independent compositions but as experimental movements in sonatas and symphonies, including the Eroica and the 9th. The form is a central one for him, and this phenomenon hasn't been investigated thoroughly enough. The Diabelli Variations are the summation of all he knew, with homage variations at the end to Handel (fugue), Bach (aria) and Mozart (the exquisite Tempo di Menuetto, itself a set of variations within the final variation), and it's all indelibly Beethoven as well.
There is a lifetime of thought and musical invention, and about a third of the Diabelli's are diabolically difficult to play, while the piece as a whole makes tremendous demands on a performer to keep its continuity intact and to reach the peaks of energy and passion it demands, particularly in the final ten variations.
For the greatest recorded performances, search out the rare Leonard Shure CD on Audiofon - the piece was a lifelong specialty of Shure's, Schnabel's only assistant, and an artist woefully (let's say: criminally) under-represented on recordings. Also find the Peter Serkin performance on LP - a rarity, but a nearly perfect performance in every regard, and technically stunning aside from anything else. Horszowski is out there somewhere too, and of course there are other great exponents, like Jerome Rose.... No one plays the Diabelli's on recording unless they're at the top rank technically and artistically - it's a summit achievement, a touchstone emblematic of ultimate mastery.
Difficulty is somewhat subjective, but this piece throws everything at you, very quickly and in rapid succession. As opposed to tackling a grand edifice like the Hammerklavier, the Diabelli's require exquisite shifts in mood, tempo and articulation, from brutal to lyrical in neighboring pages, and that triple fugue which goes into double-time at the halfway point offers a terrifying plunge into complexity and violence ...
Value? An enormous late-Beethoven piano piece that's unique - his last major work for piano, actually - can't be quantified. It's off the charts. Listen to the final page of the last variation as Beethoven says goodbye, to the world and the piano, and ask yourself what, in this world of ours, is worth more?
peace and love -
claude