Of course, there are pros and cons to foil stamping, so it's best to use the process thoughtfully. As we've mentioned, it gives a shiny, mirrored look. (Yes, metallic ink is an option — but it usually looks only just slightly metallic when printed on cotton paper.) And because stamped foil is opaque, it can be printed on dark backgrounds; it will appear high in contrast, instead of taking on some of the paper's tone and blending in, as ink would to some degree.
Onto the (yes, inevitable) disadvantages: first, it's more expensive than ink. Not all letterpress printers do foil stamping themselves, so it has to be sent out, which can extend the turnaround time. And from a technical standpoint, the edges don't print as crisply as using the regular letterpress-and-ink process — so fine detail work isn't as clean.