Intersting.. I am a leftie and often play for 10 mins or so right-handed against the wall when hitting. I did it less to learn how but to see if I could plus because I was trying to improve my overall footwork which it seems to be good for.
I personally found a good way to improve really quickly is to play short against a wall - standing only about 2-3m away. The 'feel' aspect of playing with your non-natural hand is the hardest part to learn and this helps heaps. Once you've improved that the rest seems to fall into place much quicker.
My main problem using my left is a lack of stability. My left arm feels 'floppy' and unresponsive. I think it's mainly due to lack of intensive use that my right hand/arm has had through my life.