He was a very good actor. It's a shame he never went into film. Might have greatly improved the last half of his life, after Cassidymania ended (1980).
He did a ton of TV bit parts, before and after his five seasons of The Partridge Family. And was quite successful as a music artist, especially outside the US (Britain, Australia, Canada). He had his Cassidymania. And was certainly a great singer and musical performer, but he wasn't really a songwriter, only penning about one song per his solo albums. He was broke by the early 80's.
I like plenty of Partridge songs, but I personally was not a D. Cassidy music fan. Still, he performs one of the best songs of all-time...