The Rosewill RCX-Z4 is not a heatsink for those looking to squeeze the last MHz out of an overclocked CPU. The benchmark numbers actually show that this product is not much better than the two factory coolers, but what do benchmarks really tell us besides the extreme ends of the scale. This product is really made for the 'grey' areas.
With the Z4 currently available for $39.99 USD from Newegg it would be unreasonable for anyone to expect it to perform with the Noctua or high-end Zalmans of the world. The truth is that since Rosewill included a way for the end user to adjust his or her level of performance (and with it the acoustic level), this is a really good choice for 'regular' computer users. The offset design may also come in handy when working with an odd case such as a Home Theater PC where room for hard drives or other devices will interfere with the placement of large coolers.
With that said, if you are looking for a CPU cooler that will give you adjustable performance without breaking the bank then this cooler should be one to keep in mind.