stuff 是不可数名词，所以其前不能用不定冠词。

根据《朗文当代英语大词典》：good stuff [BrE] ( =used to tell someone that their work is good ) 优秀作品。如：

This is good stuff. 这东西不错。

This wine is good stuff. 这酒不错。

This is all good stuff. Well done! 这一切都不错。干得漂亮！

There has been some really good stuff on TV lately. 近来电视上有些节目确实不错。

Readers for publishing houses see a lot of good stuff and a lot of dreck. 出版社的审稿人会看到许多好东西及许多垃圾货。