For some reason I picture Kempff in these recordings to be playing in an elegant and well proportioned drawing room that is somehow floating freely in space, timeless.
He plays Beethoven without apparent effort, as if for himself (and the universe), and he plays as a master. There is nothing showy here, nor display of technique: you would never guess that to play like this is difficult! You don't get a sense that he is interpreting Beethoven - although, of course, he is (listen to his phrasing and pacing, the way he spins a line of music, leaves it floating for an instant and then pulls it back in ... or just lets it dissolve in the air) - but that he is just playing it as something that has been inside of him forever, playing it as he finds it within himself. Kempff's playing is imperious and yet intimate; it is very fully alive and he is the master of so many different moods (the sonatas span Beethoven's life). If I say his playing is relatively relaxed, don't misunderstand me: there is not a dull moment in this set.
Comparisons with others? This is one of the great sets of these works and contains many of the great performances. I like to compare Kempff with Richter - a pianist whose Beethoven I love - because the contrast in approach is so strong. With Richter you sense deep concentration and a sense, almost, that he is improvising ... discovering the music as he plays it. With Kempff it is clear that he knows where he is going: it seems like he plays these sonatas for the same reasons that we lesser mortals merely listen to them even again and again even though we know them well.