Following some discussion here, I bought one of these in December but have only just got round to using it:
Makita 6935FDZ 14.4v Cordless Impact Driver
(bought this cheap NiCad one because I already have several Makita 14.4 and 18 volt NiCad batteries).
Making some picket-style gates using sawn/treated timber. Very impressed! It felt a bit odd the first couple of times I used it, when the vibration kicks in it's as if you have inadvertently left a combi in the impact position, or the bit is experiencing cam-out.
There's obviously some smart electronic and mechanical control gear in there, it drives in at high speed until some torque level is reached, then it slows down and the impact mode kicks in. Also *very* effective at removing well-rusted screws.
I was slightly worried whether I would break screws (being cheap I use Quicksilver rather than Spax or Turbo gold), but I did get proper "impact" bits and I didn't break anything.
Whilst I don't think it is quite as essential a tool as a combi drill-driver, you would not need to be doing much fencing / flooring / decking before it would pay for itself in time saved.