Beerboy,
I've had the Makita 12v job a bit now, and it's really something. I had reservations that the impact portion would be hard to work with....it's not! To me, drilling holes is just the same as with a regular drill. You will need drill bits that have the hex end on them. They are a bit more expensive, but affordable.
When driving screws, as you encounter resistance, the impact portion of the driver kicks in. It slows down a bit, but the impact protion is amazing. There is a HUGE reduction in camming out screw heads. The torque is incredible. I have not used it to drive into concrete or metal, but I would assume it would be just as easy. The only point of caution would be as you drive the screw, you have to release the trigger, or you'll keep driving the fastener right on the the piece your working on!
My decision came down the the 12V makita and the Panasonic impact driver. Besides cost (panasonic ~$100 more) the 'keyless chuck' for the panasonic looked cumbersome. The fit of the makita is outstanding and I have huge friggin hands. I also live close to a Makita repair station, so if something lets loose, I can go there for repairs.
All in all, I recommend the impact driver. If your work is only on woodworking, save the money and get a nice 9.6 or 12V drill. But if your at all going to use it to put together a deck, or install stuff in concrete, go for it baby!
P.S. One drawback for the impact drivers are the noise. When the impact portion kicks in, it gets kinda noisy. So the baby is sleeping upstairs, you might consider a regular drill.
Hosta, Double-Chin Tony