Being a DJ, I have heard a lot of mash ups....A LOT. Not all of them are good, in fact most of them are garbage. You give someone a computer, sound editing software and everyone is mashing up songs. They don't take in account, pitch, key, tempo and other things like that.
But, there are some mash ups that make you ask yourself "wait, this isn't the original song?" because that is how good they are.
This one below is one of the best I have heard in a while, it also will make you relive your early 2000's if you grew up when these songs were all over the radio.