(for those not already familiar with it, LDD is a sort of CAD for LEGO. It's free software by LEGO, just google "ldd")
It's still a bit flaky in places, but I managed to build my Heli Shooter that I posted a while ago without too much trouble, here's a piccy:

It doesn't have any of the 3rd party sensors for obvious reasons. I've replaced the HiTechnic sesors with LEGO ones, but left empty positions for the others.
Overall I'm quite pleased. The big advantage that LDD has over, say, LDraw is that it actually models the pieces properly and enforces them to connect in the right way. In fact, whilst putting together the sensor turret in LDD I found that a few pieces had been badly aligned and the software made it really easy to find a fix. However, the modelling is far from perfect: whether or not it lets you connect things seems to sometimes depend on the order in which you try to put them together, and there were one or two connections I simply couldn't convince it to make. It is also very tricky to get gears to mesh (yes, it even checks those!).
However, I'm probably willing to forgive its numerous flaws for sake of the fact that having proper models of the pieces makes putting them together so much easier than in LDraw.
I'd be interested to hear what others think.
Matt