I've used them, they are OK. the 600 seems to run rich at idle and stumbles when cold on a 302 out of the box.
From Hot Rod.com :
Holley's 4160 series carburetors are very similar to the 4150, but the 4160s are all vacuum secondary units, and do not have provisions for an accelerator pump circuit for the secondary barrels. The secondary side is fitted with a fixed-jet metering plate, rather than the metering block with interchangeable jets found on the 4150. The exception is the 660 cfm "Center-Squirter" carb designed for dual-quad tunnel ram applications, which has a single accelerator pump discharging into the all four barrels. With no metering block on the secondary side, the 4160 is more compact than the 4150, giving slightly more clearance for certain applications such as inline dual quads. Secondary main circuit tuning, however, is more difficult, necessitating replacing or modifying the metering plate.
The 4160 carbs were very popular as OEM equipment, and are excellent street or street/strip carburetors, with the fuel curve very well calibrated from Holley for their specified application. In usage where the requirement doesn't include frequently changing jets or re-calibration such as in serious racing, the 4160 makes for a low-maintenance but effective piece.