I like my Carradice. The Rixen and Kaul/Ortleib quick-release fitting is the one to go for. It is very quick, no faffing around with velcro or straps and totally secure so you can do descents on mountain trails.
Some cheaper mounts use a large plastic clamp to prevent rotation on the bar. R&K use a small piece of plastic-coated cable so their mount is very lightweight.