In my opinion, some of the statements by insult2injury and bheart790 need to be amended (probably due to outdated practices and ideas about valve type selection...?).
As a matter of fact, triple-offset metal-seated valves can achieve tightness performances even better than Class VI and are also used as on/off valves.
They may work fine in many different applications, from cryogenic up to high temperatures, and are more and more required by the Oil and Gas market...: please take also a look at thread408-210294 within this Forum (and at the links included there), for example.
Of course those valves are are not piggable, but they are much lighter, easier to operate and more compact than gate or ball valves (for instance) of the same size and rating.
Coming back to the main topic, triple-offset metal-seated valves should be considered as "the top" of the range which starts from "simple" rubber lined butterflies and continues with double-eccentric ones (HPBV's), including all the possible intermediate variations in the middle.
Hope this helps, 'NGL