You can use any type of value for an _id field (except for Arrays). If you choose not to use ObjectId, you’ll have to somehow guarantee uniqueness of values (casting ObjectId to string will do). If you try to insert duplicate key, error will occur and you’ll have to deal with it.
I’m not sure what effect will it have on sharded cluster when you attempt to insert two documents with the same _id to different shards. I suspect that it will let you insert, but this will bite you later. (I’ll have to test this).
That said, you should have no troubles with _id = (new ObjectId).toString().