The main areas to consider when buying real metal business cards would be thickness (.36mm is often a common weight), components, price, and the ability to personalize your cards. I would recommend copper or stainless steel instead of a cheaper aluminum card.
Prices have become more competitive every calendar year, especially when buying from abroad or directly from your reseller. Custom options, including the ability to die-cut frames and logos, are a big plus. Most metal cards were produced with "safe" rounded corners and blunt tips. Nobody wants their cards to become potentially dangerous.
Some customers may request a black, red, or another color background to print on their metal card. This creates an additional "traditional" card style while maintaining the impressive look and feel of real metal. If you want to purchase the best custom business cards, browse online stores.
Punching is typically accomplished with photochemical or laser engraving. Amazing effects can be created by downloading photo logos, frames, and silhouettes. Engraving can create stunning textures in addition to shading that contrasts with the actual shiny metallic look and feel of the rest of your card.
So are they worth the price?
In short, metal business cards are an invaluable marketing tool. To be generic, stick with your plain, boring, black and white business cards. If you need to be remembered, a true metal business card is worth the investment.