sealing a crawlspace

[benj]

i am considering sealing my very humid, damp crawlspace from floor to sill plate with 20 mil plastic. this is a company that does this sort of thing:

http://www.basementsystems.com/crawlspace.html

i currently have 6 mil plastic on the ground only, but this new solution supposedly keeps out almost all moisture (not just the groundwater), but it's very expensive. anyone have any experience with this type of all-over sealing?