Lessons from QUaD

A ground based CMB polarimeter can have excellent stability
Direct pair difference worked
Half wave plate modulation was not required
Cross linking of map not needed
Simple Fourier based analysis has proven to be adequate
By far the biggest problems have been detector weirdness and good old fashioned sidelobe pickup
Moon!

(Caveat: obviously the above are shown to be true only for the specific configuration and sensitivity of QUaD)