We've had a few problems, but on the whole we're very happy.

on 是介词，表示"在...上"或"关于"。 the 是定冠词，用于特指某事物。 whole 是形容词，意思是"全部的、完整的"。 on the whole 是一个固定短语，意思是"总的来说、大体上"，用于对整体情况做出概括性评价。