9 times out of 10, if it fits, it should work. However, there are times when the cable may be wired differently (OEMs do that just to be difficult... Sony, for example, is notorious for this).

just like the title says...my brother got an extra motorola rarz v3 charger and he saw that the end for the charger fit into my mp3...but it takes longer than usual to charge fully. So does that mean that they are not compatible?

thnx for the help guys.