In addition to the paintings and furniture, the new collection of musical instruments is well worth seeing.

well 在这里是副词，用来修饰形容词 worth，表示程度"相当、非常"。 worth 是形容词，意思是"值得的"，常与 be 动词连用表示"值得做某事"。 well worth seeing 表示"非常值得一看"，强调观赏价值很高。