Mine were rubbish - 12V 20W always failing or intermittent - no transformer issues in four years just bad connections and blown bulbs. I was a slave to my annoying downlighters but no longer.
For about £36 I replaced all four downlighters with 14" long 240V 10W miniature neons - these chaps just unplug and unclip for easy maintenance although I have not needed to change anything for the past 6 months.
My new neons have given me my life back.
Peccavi
February 2009