in not implemented

Do you have MongoDB as field storage? Or do you get this with MySQL?
I get this with MongoDB but can't decide where to fix it.

I was able to fix this problem by updating the mongodb module in "mongodb_field_storage.module" down in the _mongodb_field_storage_query_value function.

The problem is a case comparison where lowercase 'in' is being compared against uppercase 'IN'.

I changed:
switch ($operator) {

to:
switch (strtoupper($operator)) {

...and that worked for me.

Someone should commit this update...

I provided that patch already in february in http://drupal.org/node/1459116 But it was not included. Probably you could go there and give more feed back why it's needed?

I will fix it over there.

Hrm, I was wrong :) the documentation clearly says IN and so I will fix it here.

Here's a quick and dirty patch that fixes the problem for me.

Thanks for the patch but this IMO should be fixed in the IN handlers themselves (handlers/efq_views_handler_filter_entity_in_operator.inc, handlers/efq_views_handler_filter_field_in_operator.inc, handlers/efq_views_handler_filter_property_in_operator.inc)

How about ALL, STARTS_WITH, CONTAINS and BETWEEN, should we open separate issues about them and their case or include in this one?

Check the alpha. ALL is not supported by EntityFieldQuery, STARTS_WITH, CONTAINS and BETWEEN is implemented.