I think the Shimano freewheels are fantastic. I tried a Nashbar knockoff since it had better gears to pick from, but it doesn't shift as nice, and sometimes makes noise. First thing I do to a bike, when I rebuild one, is upgrade to a Shimano freewheel. Second thing is take some black paint or a magic marker to the logo.

Shimano is the way to go. I'm sure IRDs are nice, but with SHimano's at $10-15 it would never occur to me to pop $50-70 for one.