The first time I ever saw a fireplace tiled with mosaic glass tiles was about two years ago when I was house hunting: I walked into a 1950s ranch-style bungalow and the fireplace had been tiled with leftover tiles from the recent kitchen remodel. When I moved in, I initially hated it: the colors were off, the mantel was painted a dull silver. In my mind, I wanted to restore it back to its initial glory with its original brick surround and oak mantel. But after putting off the project month after month...
...I got used to the tiles (the mantel, however, had to get repainted). Now, as I look at my fireplace, I'm actually really glad that I didn't chisel the tiles out: it adds a little bit of character to the living room, a nice unexpected twist from the usual slate or brick surround.
If you're thinking about doing a fireplace upgrade and haven't considered doing a tile job, here are a few photos that may inspire you to consider the look:
And a close-up: