I can definitely vouch for cast iron.  I've had a large pot and a large skillet for the better part of 10 years, use them all the time, and they are still in near-perfect condition.  Copper does last, but cast iron actually gets *better* with age; after a few years of steady use, cast iron pots and pans will be so well-seasoned that *nothing* can stick to them.

Cast iron is:

  * Dirt cheap;
  * Long-lasting;
  * Easy to clean (just rinse)
  * Oven-safe (you can use it as a dutch oven, or for [slow cooking][1]).

As long as you don't mind the weight, and you don't let them rust (dry and lightly oil them after rinsing) you can't go wrong.  I know some cooks that swear by their copper cookware, I know it has a lot of great properties, but considering the cost, I really think that cast iron is the best investment if you're looking to "upgrade" your cookware.

  [1]: https://cooking.stackexchange.com/questions/488/slow-cooking-without-a-slow-cooker