My boys go to a school which has mixed classes. approximately 15 children in each year, reception class is stand-alone, and then year 1+2, 3+4, 5+6.
I have nothing to compare, but my (and my children's)experiences are mainly positive.
Eldest just completed his first year in secondary school. He is very bright and able, and never seemed to have any problems with being bored or held back.
Middle child just completed year 4, has some literacy problems, but has received good support from teacher and SENCO.
Youngest child just completed year 3, will cope anywhere.
Most children and parents are content with the school. Obviously issues raise their head, but rarely about the mixed classes. Parents are usually anxious when they start, because they don't see how it could possibly work, but then appear quite happy with things once they've seen it in action.
Big plus: the children makes good friends across three years, rather than just their own year - very useful once they go to high school, especially as they tends to disperse to several different schools.
Big minus (for us) two lads in the same class every other year can lead to excessive competition between them, lol. Wasn't a problem when they were year 1/2, but has been noticeable in year3/4. They've got a year's space now until they're together again.
It CAN work, and it can work well.