They're a very competent bunch of musicians who play pretty music that leaves a taste of plastic in my mouth. It bears no relationship to bluegrass, they don't use any bluegrass techniques or style (no banjo, for a start!). The occasional Old Time songs they play don't seem to have been learned or influenced by listening to the old folks.
Anyone who's not heard them can hear loads of samples here I'd classify them as a polished acoustic pop band.
Cheers
Ray