This latest video of Mixxx 1.6.0 in action demonstrates what scratching sounds like with vinyl control. In the video, I'm scratching with a Serato record on my Stanton T.80 turntable, and using my M-Audio X-Session Pro MIDI controller as a virtual mixer. I changed Mixxx's crossfader curve setting in the preferences to "fast cut", which is what you'd set the curve control on a real mixer to when you're scratching.
The turntable is plugged directly into my laptop using the turntable's line-out and my laptop's onboard soundcard.
I recently did some work on our "vinyl emulation" code that mimics the way the pitch changes when you're using a turntable. I managed to improve the sound quality enough that scratching with Mixxx + vinyl control sounds convincingly similar to scratching with real vinyl. These improvements will be included with our next beta release.