Overclocking is not an exact science. For example, every processor is different and just because your friend's Core i7 processor hit 4GHz on air doesn't mean that yours will, even if using the same settings and hardware. Many factors can influence what a processor is capable of. These factors include complementary components like the motherboard, memory, power supply and cooling. In addition, user experience definitely comes into play as there is an abundance of modifiable settings within the BIOS.
We were able to reach a higher BCLK with Gigabyte's UD6 than any other board in the round up. With a setting of 206 x 21, our stock Core i5 750 processor maxed out at a stable frequency of 4.3GHz. The majority of motherboards in this comparison group found stability at a BCLK of 203, but we could not get Intel's DP55WG to make it past 172.