I have a propel and the caliper's are very good, much better than I expected and equally on par with my Ultegra ones. However the first thing I did was swap out the supplied shimano brake pads for some swissstop pads.
If they're not springing back when you release the lever then as Stonechat said I would check them for crud. I find that because of the placement and close proximity to the wheel the cable that bridges the wheel gathers all sorts of muck. Get a baby wipe on it to clear it if it's filthy.