My id girls started in reception this year and are in separate classes, which is what I wanted as well as being school policy.
Even in nursery, they had their own separate 'best' friend, although they did play together quite a lot as well. But I was aware that one of the teachers in particular wasn't making much effort to distinguish between them and called both of them using both of their names IYSWIM (DT1nameDT2name).
In general it has worked well - DT1 was put in the same class as her best friend, but unfortunately DT2's best friend went to a different school, so although she knew children in her class from the nursery she missed DT1 a bit to start with. The school were great though - the reception classrooms are interconnecting and unless the classes are having circle time, the door is open and the children are generally free to move between the classrooms. Occasionally in the first few weeks, if DT2 was struggling, she would stay with DT1 for circle time. They now have their own definitive circle of friends, but also play together a lot.
The only slight issue I now have is the difference in teaching styles - both teachers are excellent, but when you have a child in both classes, you can see which is more effective. Of course when you think about it, this was always a potential by-product of separating DTs and I am just relieved that, for now, I don't have the problem of one excellent and one not-so-good teacher. That could happen - there are less effective teachers in all schools - but I suppose on the positive side, at least I'll be more aware of it, and able to compensate for any short-comings at home and/or kick up a fuss at the school .
I believe separation is more important for id twins - teachers and children can find it hard to tell my two apart and I wanted them to develop as individuals as much as possible instead of simply being seen as one half of a pair. A friend has non-id boys and it's not obvious that the boys are even brothers, let alone twins she'll be separating hers anyway, but I feel the implications of not separating them are fewer. I also didn't want my girls being constantly compared (it's hard enough not to do that as a parent )...I know that their teachers do discuss them (they discuss all the children), but there is less direct comparison made now that they are in different classes.