I like to have 2 machines on hand for every T&G job of any size: my Challenger with the standard T&G brush, as well as my Daimer steam vapor machine (SVM). (http://www.daimer.com) With its 300+ degree heat at 70 psi, with a brush attachment that's just a bit wider than a grout line, it can blast out whatever crud is lurking there. Sometimes I'll do the entire floor with my Challenger, then go back over the lines that still look gray with the Daimer. I wrap things up with the Challenger and terrycloth pads to remove all the mess. Customers love the results!

I learned the hard way to be patient, let the products dwell. The right products plus weight on the challenger, plus suction to remove the suspended dirt have worked for me. I haven't bought a steam unit yet but to Mark's point it's the contrast from grout to tile that makes the customer happy (IMHO). If I find a small used unit I might follow the Professor's (Dr. Dullea) lead.