I bought a Makita BTD 141 a month or two ago. It is a fantastic tool for sinking screws, lag screws, etc. I recommend it to everyone I know.
I've not tried to drill any holes with it, as I don't have any 1/4" hex shank drill bits. But it seems to "wiggle" a little, so I'm not sure how it would drill.
I've got an old Porter Cable cordless drill that just won't die and a Bosch hammer drill that eats anything I throw at it, so I'm content with just driving screws with the impact. And it does that VERY well.
It IS loud, but no louder than the hammer drill when hammering into concrete.
I say buy both the impact and the drill/driver. They are two different animals that can overlap a little, but each has its strengths.
Check out this thread for some more opinions/info.
And don't forget, you can never have too many tools.