My DS was in composite classes all through primary. In his case, the school only had composite classes due to the small numbers, so the whole place was geared up to it, which I think may be better than schools where there is a mix of single age and composites.
In his school, the make up of the classes would change each year depending on the numbers in each year group, so sometimes all of his year group was in the same class, and other years they would divide between two classes (they always did that strictly on age so there'd be no argument!).
It worked very well for him - primary classes are usually being taught in ability groups for things like maths and reading anyway, so it just meant the ability spread in the class was wider, but they were still each in groups for their ability IYSWIM. For other subjects like projects etc, they would all be doing the same topic, but the teacher would expect more quantity and detail in the older years projects, or would set more complex worksheets etc.
Are you in Scotland? If so, another advantage is a maximum class size of 25 which compensates for the wider ability spread. Not sure about England though.