Is this the first time you have used that tin? and What type of oil did you use?and, are you sure that it was metal flaking off or could it have been the 'non-stick' coating? (I agree, gross either way)
I have a silicone muffin pan and I love it. However, because it makes 'texas sized' muffins, I don't use it that frequently. The tins I use most often had belonged to my grandmother, so they are really quite old.
I make muffins often, at least one to two times a week. I keep a varity in the freezer for dd2's lunches and for snacking. (My muffins are also gluten free).