It depends on whether or not those iPhone users have turned off iMessage out of some misguided sense of protocol purity. If some but not all have it turned off, their experience will be similar to an android user in a chat with iMessage users (ie, terrible).

If everyone has it turned off, it is just the normal group SMS experience (also terrible). SMS group chat isn't gone, you just very rarely see it because there is almost no reason for an Apple user to disable iMessage.