i also use the rimmel one, which is very good, though MAC have recently bought out a black pen-type one which is meant to be good.
Like everyone else says - practice. I'm right handed, and used to have terrible trouble getting my "flick" as good on my left eye as the one on the right, but i can do it really well now,. if you get most of the eye right, and the flick goes a bit wonky, i find a cotton bud dipped in a bit of moisturiser will erase any mistakes, and you can try again.