Overclocking is not an exact science. For example, every processor is different and just because your friend's Core i5/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. Overclocking is, by its very nature, unpredictable. Even if you buy a CPU according to make, model, and week of production, there's no telling how much additional overclocked performance you might or might not get.
The Asus P7P55D-E Pro is a decent overclocking board, but we were less than thrilled with its stability in certain situations. Our Core i5-750 was stable up to 3.5GHz (175MHz base clock), but the board become unstable above 180MHz, even after we increased voltages and adjusted settings in the BIOS to compensate.
This is an area where the E Pro's stability could use some improvement. The system had no problem looping benchmarks when running at a default 1333MHz, but became unstable after we pushed RAM speeds up to 1600MHz. This instability persisted even after we'd significantly raised RAM timings, increased RAM voltage, and swapped some of the Elpida DIMMs in the board for identical DIMMs we had handy. We should note that we've previously qualified these DIMMs as 1600MHz in multiple Core i5/i7 motherboards, but for whatever reason, the P7P55D-E Pro and our RAM weren't fond of each other. We then switched to a set of three Corsair DDR3-1600MHz DIMMs and had no problems, though we lacked a fourth DIMM to make a fair comparison.
The MemOk! button was no help during these tests--it may be designed to resolve RAM timing instabilities, but evidently our Elpida RAM didn't kick back any issues until benchmarked heavily.
We wouldn't recommend the P7P55D-E Pro as an overclocker's dream board, but if your needs are modest and you just want an extra 300-500MHz, it's a fine choice.