Some say that cooking acidic foods in cast iron will impart a metalic taste, but others (including me) say that if your pan is seasoned properly, that won't be a problem. Here's an old thread w/some detail on the debate: http://chowhound.chow.com/topics/286179. I love my cast iron skillet and use it for most everything. I do wash it with soap and water - scandal to some, but I often cook fish/highly flavored foods in it and I don't want my pancakes to taste like curried salmon, for instance!
I think if you clean it properly and season it you will never notice it. I'm sure lots of chowhounders have worked in restaurants that use/used cast iron and never once had a dish brought back to the kitchen saying that it taste like iron. I'm also sure that you can find people who can detect aluminum and stainless steel too. You can always use salt to clean it after fish. As a last resort re-season it.
I you are a prospective buyer of cast iron for the first time I don't think it should be a reason not to try it.
Iron, maybe slightly, though I've never noticed it; "old food", certainly not. The only situation I can think of in which that might have an effect would be frying fish; if the oil tastes strongly fishy, the amount that seeps into the seasoning on the pan may provide a very slight fish taste. I've never fried fish in my cast-iron pan, though, so I can't say for certain one way or the other on that.
Depending in the food, yes, The Iron does give a taste. The cooking in a cast iron pan does provide a nutritional source of iron.
"Health Benefits of Cooking with Cast Iron
While cooking with cast iron is known for increasing the daily source of iron, it is important to understand that it can be stored in the body and can accumulate over time. Most American’s don’t get enough iron and it is also lost through perspiration, especially people like athletes who perspire heavily during activities, and using cast iron for cooking can help to re-supply the iron. However, it has been suggested that iron supplements should not be used without consulting a doctor."
From article at http://healthycooking.suite101.com/ar...
Things that are fried like eggs, bacon or deep fried like chicken to me have less to no iron taste to me. YMMV.