I'm using the cab clone built in to my MV25 to practice at low volume and I love it. I actually seem to be getting more low-end that way than with the external speaker. Could be my monitors, though, as they are pretty thumpy.
I want to get a standalone cab clone to replace my Radial JDX for my other amps. Having the dummy load and no wall-wart seems like win-win.
My Rock Crusher does a good job as an attenuator, but the tone coming out of the line out is terrible. Totally unuseable. The JDX sounds great, but it also loses a bit of low-end. Like Authorized mentioned, easily compensated on the console. I have been using it for FOH and making my Mini Rec sound HUGE.