On 12 January 1782, Mozart wrote to his father: "Clementi plays well, as far as execution with the right hand goes. His greatest strength lies in his passages in 3rds. Apart from that, he has not a kreuzer’s worth of taste or feeling - in short he is a mere mechanicus" (Latin for automaton or robot). In a subsequent letter, Mozart wrote: "Clementi is a charlatan, like all Italians. He marks a piece presto but plays only allegro."
The main theme of Clementi's B-Flat Major sonata (op. 24, no. 2), however, captured Mozart's imagination. Ten years later, in 1791, Mozart used it in the overture to his opera Die Zauberflöte (The Magic Flute). This so embittered Clementi that every time this sonata was published, he made certain that it included a note explaining that it had been written ten years before Mozart began writing Zauberflöte.
Though he composed symphonies as well, he's still most famous for his piano sonatas (over 150 - some for the harpsichord, actually). These were often found on Beethoven's musicstand. Clementi also composed "Gradus ad Parnassum" Op. 44, which is a series of 100 short pieces for the development of piano technique. Personally these are a bit dry for my tastes but they are very impressive. As far as the sonatas, Schindler writes: "Among all the masters who have written for pianoforte, Beethoven assigned to Clementi the very foremost rank. He considered his works excellent as studies for practice, for the formation of a pure taste, and as truly beautiful subjects for performance".
Here's Clementi's Sonata in F, Op 33 No 2 II mov Allegro con fuoco:
Muzio Clementi Wiki