Mashups, for me, are really hit or miss. I have to be in a really, really, REALLY weird mood to want to listen to a majority of mashups. Recently, I've been on a "bit" of a kick, listening to almost any of these blendings that pop up on my various newsfeeds and timelines. The most recent of these to have appeared is Isosine's "Sympathy Song," mixing the oh-so-famous Ellie Goulding's cover of Elton John's "Your Song" as well as the not-so-famous-but-still-oh-so-awesome Metric's "Gimme Sympathy."
Isosine takes the simple, elegant, and emotional piano from "Your Song," as well as some of Ellie's oohs and ahhs, adds Emily Haines' powerful vocals from "Gimme Sympathy", and creates one of the neatest mashes I've heard. I really don't have too much more to say on the subject. Just that this song is one of those I didn't know I needed in my life until I heard it.
Isosine has released all of his downloads for free on his website. I highly recommend "Intro (No Power)" from Mashup Manifesto II, which also includes a Metric sample!