It is still an option, but limited schedule makes it a rather unappealing one (IMHO). Check it out: http://metrarail.com/content/metra/e.../schedule.html. Blue Line may be slow, but does not involve boarding ATS to Economy parking, then catching a shuttle bus to Metra, then waiting for the next NCS train.

BTW, there is a fairly frequent bus service from ORD to MKE: http://www.coachusa.com/wisconsincoach/.