I use Klean Wall (Non HF). Dollar store toilet bowl cleaner works too. They get dull from overspray, made obvious by the way it's only on the side of the vacuum that faces the bays.
Since they're under a shed, I wipe a couple of them down every day, and they stay pretty clean, took ten years for a noticeable haze to build up on them. I tried Shiela Shine and Sprayway and a couple other stainless "cleaners," but all they seem to do is leave a film as Greg said, then the first jackass that comes along and beats their floor mats on the vacuum leaves it completely caked in dirt. The stainless cleaners don't remove anything that a damp cloth can.