Right gents, I have completed my project (new ceiling, to help reduce sound travelling from the floor above) and have extensively tested out the Hitachi set.
The hitachi set performed admirably.
I was using some very very long screws indeed, though not all of the screw was screwing through timber - part of the screw was sitting in timber with a drilled hole. What I can say is that the impact driver had no problem screwing about 150mm of 6 x 200mm screws. Approximately 90 screws were used, out of which 2 got stuck, ie. they wouldnt unscrew or screw - they jammed in the wood. Perhaps if I had access to an 18v impact driver, this tool would've been able to turn those screws.
The impact driver is very small and compact and is ideal for screwing screws in cramped positions. The tool has a very high quality feel to it and performed without a hitch during the project.
The cordless drill driver was VERY handy. throughout the project, although I had access to a big/heavy SDS drill and an electric drill, I hardly touched these, when drilling through wood. The 10.8v drill driver is very compact, is easy to handle and quite powerful, when drilling through wood. Obviously the choice of drill bit is important, but I was impressed with the performance of the drill driver.
Though the electric drill has a MUCH higher RPM, the 10.8v drill driver is just so convenient to use, due to its small size.
I can see that in the next 20 years or so, all tools will move in the direction of getting smaller and lighter (like the Hitachi), while maintaining the outright power of some of the bigger tools available today. For the professional who uses these tools year round, I can see massive benefits of using these small tools, over the larger tools, even though the larger tools offer more power.
I would rate the Hitachi set, which I bought for around £90, 9.5/10. The only reason why I knocked off 0.5, is because the impact driver was unable to turn 2 screws which had managed to get stuck in the timber. For £90 though, I honestly can't see a better deal out there right now.