Szymanowski is in my pantheon of favorite composers. I like him mostly for his middle-period works, which have a tendency to be very lush, although not without irony or bitterness. I also appreciate how he saw composition as a moral dilemma and sought to see music as something transcending personal perspectives--thus he strove to find a musical language that uplifted his country, taking the historical folk music of the Tatras as a seed from which his modern vision bloomed. I don't prize his nationalistic works as much as his middle period ones, but they are exceptionally well crafted. His Op. 62 2 Mazurkas are to me an advancement and a synthesis of his "inner" middle period and his "outer" nationalistic period, and I wish he had lived longer to develop along these lines.
With Szymanowski you always get craftsmanship, a mastery of compositional technique from counterpoint, harmony, structure, to melody. His lyrical sense was special, borne out in many exceptional songs and his opera King Roger, a personal favorite of mine. He was a complex person who battled many hardships and lived in a terrifying era. He was very conscientious about himself, always keen to improve his awareness of self and society. This conscientiousness often took a form of a questioning. At any one of his periods he could have rested on his laurels, but he attempted to always evolve and refine his works, often with drastically different ends. This explains to some extent his relative lack of popularity, as his three major periods have quite distinct aesthetics, although there is, upon inspection, overlap. He was a great synthesizer, with his early period influenced by Chopin, Scriabin, Brahms, Wagner, and Reger. His middle period, which is his so-called "impressionistic" period, marries the harmonic intensity of Scriabin with techniques from Ravel and Debussy, and contributes much to the style, both compositionally and in terms of sophistication of sound, to me representing an apex of the movement. His nationalistic period was influenced by both abstract music and neoclassical composers, notably Stravinsky, with whom he once debated over the nature of the piano, Stravinsky saying that was essentially a percussion instrument, but Szymanowski insisting on its illusionistic capacities and melodic potential.
There are only a handful of composers who I wish could've lived much longer, because their work is capable of responding to new stimuli and making new sound-worlds. Szymanowski is one of them who, having accomplished so much, could have done so much more to advance compositional form and modern forms of expression. His work is hard to take stock of, because it is so varied, and admirers of one period may have distaste for another. But it's masterfully conceived and a musical corpus that I personally treasure.