I'm cooking a pot roast as we speak in a Le Creuset 7 qt. enameled dutch oven. FMB thinks Le Creuset is overpriced and he may be right (I got this piece in a sales promotion) but this really is a remarkable piece of cookware. Even, consistent heat, very durable and cleans up easily.
I usually put tomatoes in a midwestern chili that I make in a regular cast iron dutch oven, haven't detected any metallic taste. Might be different with a marinara sauce, though. In chili the tomato flavor is surrounded by lots of other stuff.
If there is a World Market store in your area, I've seen some reasonably priced enameled cast iron there.
http://www.worldmarket.com/home/index.jsp