I just bought regular oven cleaner at the dollar store - odor free. I sprayed it on my shower walls and ta-da off came the scum and yuk! I have a tub surround. I forget what everyone said about marble tile - I don't think they wanted to risk it.
The only problem I had was the fumes - even odor free. I had to hold my breath, spray and run from the room a few times. I don't have a window in my bathroom.