I play them (not yet all 51 of course!!! )They're really hard stuff but extremely helpful
very high, because they should not be a torture.but you don't have to be a perfect pianist (not yet )I use them usually as a warm-up before practicing.As a level: You should be able to play the technical difficult Beethoven Sonatas (such as 2/3; Waldstein etc.)
The Paganini Variations are far harder than the exercises. You can't really compare the two, but I can say that the Paganini Variations are probably the hardest thing Brahms wrote, and one of the most advanced pieces in the piano repertoire.The exercises themselves are very challenging but are helpful, once you've worked at them for a bit.