Our ceiling is fairly well insulated with blown cellulose, and our walls are double clad weatherboard, which isn't terrible. But our floor is timber floorboards with an R value of probably 0.2. Several times with several people I've discussed underfloor insulation. Today I tried rockwool + sisilation, which was the cheapest per R value method we could come up with. It was actually quite easy, though with two people it would be a lot easier. The batts are already cut to exactly the right width to fit between the joists so I just had to tuck them in. Surprisingly, they actually stuck in position for the most part, so I then just put a batch in to the width of the sisilation and stapled that in place (yes,avoiding the power cables).
The main problems are a) the cramped conditions, b) the dust c) the little rockwool spiky bits which stick in your skin. But mainly I suffered from the dust from the ground getting in and on everything.
I've only done one room, our computer/tv/reading room. But given the ease I think I'll do the whole floor once I can convince someone to help me. The batt bags go a surprisingly long way, the whole room only required a bag and two batts.
Possible future problems:
Rodents in the batts. Not sure how to stop this, other than have cats. Hopefully the floor is high enough to stop them getting up there. Also, the sisilation may be sufficient deterent.