These non-nuke density gauges are new to us as well, and we are considering purchasing one or two - I think our plan is going to be to use the nuke and the TDR units side by side for a bit to see how they compare in differing soil conditions before we make a firm move one way or the other.
This has become an issue for our firm since we are expanding and it has been a major hurdle to get licenses for use of nuclear gages in other states. We will be running trials with the TDR method for moisture-density measurements
If you want more information, I would recommend that you plan to attend TDR2006
There are continuing developments with this technology beyond measurement of soil moisture.