I just cut a hole for a 120mm fan in a friends window on his case using a roto-zip and it worked unbelievably well, like cuting through 1/2 melted butter actually. I could cut as fast as I wanted without melting the acrylic and it created no cracks whatsoever. just for a test after I was done I used my dremel tool with the same bit on a piece of scrap and was surprised to find that it didnt work near as well. I figured using the same bit would have made it work just as well, but it didnt. Also, as mico said there was no need at all to sand the cut after it was done, I had my file and sandpaper out and waiting, but was pleasently surprised when I realized I didnt need it. Also my rotozip came with a circle cutting tool thingy which only helped to make it even easier to cut the hole.
And by the way the roto-zip works just as well on metal as the acrylic.