3/4's are a bit pointless if you ask me. Why have just 5" of ones leg uncovered, same for knee warmers, hence why I seldom wear mine.
20°C and under, legs covered.
As for leg warmers, Giordana make some good ones, but then again so do Adidas, Bioracer and BTWIN.
Try before you buy, some are way to short so can slip down easily.