I have tried AirDisplay with the new iPad; it's a bit laggy but works OK in 'normal' mode, although everything displays very small on the iPad display.
On my configuration (Mid 2010 iMac with i5 CPU, 4GB RAM + new iPad), HiDPI was so unreliable as to be unusable. It would work for a few seconds then display a blue screen on the Mac and then dump me back into normal mode. I worked with the developers for a few days trying to troubleshoot the problem (they provided good support BTW) but they didn't have a similar Mac and were unable to reproduce the problem.
I ended up asking for (and receiving) a refund.
Shame as it looked to be a good idea. It's possible that it's just a problem with my particular Mac configuration so YMMV.