Nearly all DJs today - Rock or otherwise - have found that many fans respond well to having two songs moulded together to create a single intertwined song. This is mostly used in the hip-hop scene where it is better known as mixing. But now some Rock fans have taken some of their favourite songs and created a mash-up.
Most often these are done well, with the songs flowing through each other seamlessly. The upside of mash-ups is that it can provide you with new versions of your favourite songs or even combine two of your favourite songs together to make a supersong.
The downside is that this can reduce the enjoyment of the song overall as you may have the opinion that songs should be left the way they are and not tampered with - excluding covers by other bands. Also, the mash-up may not be done well and the product is quite literally a messy mash-up of two-or-more songs.
Sometimes a person takes things way too far and tries to stuff 6 songs into a single 6 minute "song". I swear, I've seen a mash-up that includes Korn, Limp Bizkit, Eminem, Linkin Park, Papa Roach and some other rapper, which is complete rubbish in my opinion.
I'm a Punk/Grunger, so some mash-ups really come out nicely, even though they may look weird on paper. Songs such as "Boulevard Of Misery Business" by Paramore/Green Day or "Smells Like Song 2" by Nirvana/Blur or even "Smile/Homecoming" by Avril Lavigne/Green Day. And EXTREMELY surprisingly "NEver Gonna Give Your Teen Spirit Up" by Nirvana/Rick Astley which sounds hopelessly stupid but there was something really catchy about the chorus.
Another really horrible aspect of mash-ups are the extremely dense names given to the mash-ups. Such as "Hey We Will Rock You Yah" by Queen/Outkast.
Where do you stand on mash-ups and do you have any personal favourites you would like to share?