Quickest way to get flight quotes

Hi all,

I have been using the BoardingPass program to compare flight prices
across several web sites, but I have never actually booked anything
using the program. Can anyone share any positive/negative feedback on
booking flights through BoardingPass?

I guess the more general question is this: which sites or software do
you recommend to quickly search and compare prices on flights?

I am based in the UK, but fly often to the US, and domestically in the
US.

Thanks for your contributions,

Tim