I think I have used and evaluated every cordless impact made for work since we use quite a few on the assembly line. This is what we use alot of of and what I have at home. Probably the best price-power-weight-size of any cordless out there. 3.5 lbs with battery and puts out an honest 160 ft-lbs as tested at work on our impact tester. There are stronger ones out there for sure, but they either cost more, weigh more, or both.
http://www.toolbarn.com/product/hitachi/WR18DL/
Cheap ones have crappy batteries that don't last and put out way less than advertised torque.