I also have the $50 Sony Fontopia ones, and I can say that they allow me to listen to my music back and forth on the bus to work every day without having to turn up the music beyond what I do at home! Sound leakage is pretty minimal at the volume I use--if I take them out of my ears and hold them a foot or so away I can't hear them at all.
They DO screen out external noise to a fair extent too, though, so you wouldn't want to wear these in an environment where you need to hear--conversations with co-workers, your boss suddenly barging in, etc.
You also really do have to get them in there, and get a good seal, for the sound to be right. The instructions recommend pulling your earlobe out a bit to slip them in, and if you still can't get a good seal, to try moistening the silicon. They aren't kidding, either--I routinely have to use the ear-pull technique to get them properly in far enough.
I'm very intrigued with Apple's version, given the quality of their ordinary earbuds for the price...too bad this would be even more unsanitary to try out in the store! 😛