The first mashup that I heard was probably the one "proving" that all Nickelback songs sound the same. It was impressive to me then because I was young, I have no personal musical talent, and the experience was new to me.
Since then, friends have introduced me to the "Top of the Pops" Germany-Mashups, like the 2012 Pop Mashup. The editing is unbelievable, but the video contains profanity and suggestive images... just like the original songs. What's inspiring to me is that artists deeply embedded in our "remix culture" have taken the art to the point where they can mashup not just audio or video... but both at the same time.
They take popular, well-produced media and create a new, equally well-produced product. They use the elements from the original "stories" to create their own.
I've already discussed how you can get excellent editing practice by cutting to techno. I'd recommend cutting a mashup if I didn't think the process would be so insanely frustrating.
If you're up for it, and you're willing to risk current copyright law, I'd love to see what you produce.
Your Media Production Mentor