I have run into situations where I cannot muscle off a bolt, such as a brake bolt located on the inside, with not a lot of room, I even used a breaker bar. (I am doing a brake job on a G35) I have heard some impact drivers(wrenches) are not worth it.
Anybody have one and would they recommend it? Also its pretty tight behind the brakes and so I am thinking a cordless might be too bulky and I won't be able to get a hold on the bolt with the impact.

 

well I would probably recommend a right angle impact wrench, air powered. there are electric ones that are extemely powerful but inherently large, and right angle ones can't pack the same punch.

 

I bought a small (130 lbft) impact gun from Campbell Hausfeld. It works great. Haven't had a problem getting anywhere I need to be. I also picked up a pro 65 ftlb rachet. It worked perfect when I did my HFC. Most people say you need to get to the top cat bolts from the engine bay but I did it from under the car with my impact gun and the rachet. The whole job took about 1.5 hours start to finish.

EDIT - Also definately stay away from the electric guns of all types. In my experience they are totally useless, too heavy, hard to control and bulky. Get an air compressor, it will be the best tool in your garage.