I bought a small steamer similar to the Steam Bee. It performs quite well, but did not remove soap scum from a shower screen, neither did it remove much from the oven. My view is the steam cleaner will not do much more than a conventional cleaner and good old fashioned elbow grease.
I have been told they are very good for cleaning alloy wheels, but have not tried it myself.
I seem to remember there was a previous thread about steam cleaners on AAM.