The dominant vendor today is Clear, aka Verified Identity Pass, which charges $128, which includes a $28 TSA "vetting fee". At present, there are 13 Clear airports, with 4 coming soon, including my home town of Denver, which is supposed to have its RT lanes open by the end of January.

Unfortunately, I have not yet had an opportunity to use my RT card. It seems that all of the places I have been traveling to don't have RT lanes! I was excited to find out that Clear has lanes open at La Guardia (where I'll be traveling in a couple of weeks), but then I discovered that they only have lanes open for certain terminals--and my terminal isn't on the list.
I will definitely use the Clear lane at my home airport when it opens later on this month, although it won't speed up my experience at DIA much, as I hold Premier Executive status with United, which allows me to use the 'premium traveler' line anyway. The real decision point will come next August when my Clear card comes up for renewal--if the only airport that I've been able to use my Clear card is DIA, I probably won't renew the service for another year.
It's a great idea, but until RT lanes become ubiquitous at every airport in the U.S., their value will be limited, particularly for frequent travelers such as myself that can already cut security lines.
** News Flash ** - Clear and Hilton announced on January 25, 2008 that, effective Feb 15th, Hilton HHonors program participants will be able to pay for their Clear membership with 35,000 Hilton points. That's quite a few points, but I guess it beats laying out the $128 in cash.....