Pros? Bragging rights and an extra kick out of a cheaper processor.
Cons? Excessive heat, voltages the processor wasn't designed to handle, instabilitly, shortened lifespan......I could add a dozen more to that list.
Let me explain a few things. Processors are made in batches, on one die. Ever seen those big, round pieces of shiny silicon? That's a couple of hundred cores. Not all of them are capable of running at the same speed. You'll have some that can run stably at 2.66ghz, some that can only run at 2.1ghz, and a very few that can run at 3.6ghz. There's no fundamental difference between, say, Lynnfield cores in Intel chips, other than the fact some of them can blaze right along, others can't. The slower ones are locked down and sold for cheaper prices to maximize dollar to manufacturing. AMD goes by bins, Intel does things a bit more precisely, but not much more. If you've taken the time to read up on this, you'll see that hardcore overclockers aren't just buying one brand and model of processor, they're looking for very specific batch codes. Those have a reputation of coming from a batch that was clocked down due to the majority of the tested cores being capable of a slower speed, but had a good number of cores that could, indeed, run much faster. Other batch code processors are far less successful at overclocking, so they're not sought after. The actual "holy grail" of overclockable processors are engineering samples. I have one. I'll explain that in a sec.
Intel and AMD processors, both, are multiplier locked. Back in the old days of the P3, the multiplier was unlocked, meaning you could adjust the clock speed frequency to the FSB, speeding up the clock, without raising the FSB. That had the potential of taking a 600mhz processor and jacking it up to 900mhz, without much further tweaking, as long as the BIOS supported that adjustment. Since the P4, multipliers are locked, and cannot be adjusted manually. New processors can clock DOWN, as long as the processor supports SpeedStep (on Intel), effectively slowing the processor down under light usage for power and heat savings. That feature can be turned off, locking the processor at full speed at all times. I leave mine on, it'll throttle me back from 2.66ghz to around 1.2- 1.5, depending on usage, when doing light work like web surfing, text editing, stuff like that, then crank me back up when I fire up a game or my DAW software. Otherwise, you can't adjust them to actually make the processor run faster than the rated speed. The exceptions to this rule are AMD's Black edition processors, and Intel's Extreme procs. Those are unlocked, but you pay the price for that. Not cheap. Both manufacturers are going to give you the opportunity to go faster than the locked versions, but they're not going to sell them at the same (or cheaper) price.
Cooling is EXTREMELY important. Given that all you can do now is raise the FSB, you're forcing the processor to draw more power, heating it up faster, and run hotter. If you want any kind of stability, you need a high-quality aftermarket cooler. Stock Intel or AMD coolers will not work well beyond the stock speed. I'm sure you've seen guys running newer processors at records of 6.5ghz.......they're not using stock coolers, and they're not using air coolers. For the most part, they're not even using watercooling, they're using liquid nitrogen, TEC's, or even a combination of the two. Reports often gloss over that fact. It's vital to have the best cooling you possibly can. That's where the bragging rights come in. Sure, you got 6ghz out of a 2.66ghz proc, saving money over the Extreme Edition.......but you just dropped a grand on the cooling system to do that in the first place, and double the money on a motherboard even capable of going that far. You're not going to get a 1ghz overclock using a Corsair H50 $70 watercooler kit.
Motherboards play a big role, as well. Get a cheapie Biostar mATX, you're not going to get very high in speeds. Get a top-end Asus or Gigabyte, you'll go much farther, at approximately triple the cost. Along with the BIOS options for the processor, you need a board that can lock the PCI/PCIe (and AGP, back in the old days) buses to their stock speeds. Not a good idea to crank a 33mhz bus to 100mhz.......not if you want anything to work for very long.
RAM, as well. A-Data value RAM won't run stable past what its desiged for. It can't. That ties in with the FSB you'll raise to get that CPU speed. Raise the FSB, you speed up the RAM. High-quality, overclockable RAM is absolutely essential, or you're dead in the water before you even touch the processor speed.
PSU. Need a good one, and a powerful one. Don't even think about putting a bare minimum, low powered CoolMax or Sparkle in your PC, then crank the juice. That underpowered, cheaply built PSU will start wobbling all over the place under the power draw, if its even particulary stable to begin with, and you absolutely must have rock-solid voltages. No exceptions, no margins.
That's a lot, but pretty much covers it in a nutshell. Even with the best parts, you don't know how far that cheaper processor can go until you try. You could get lucky, and get the one core on that die that slipped through the inspection gap, but more likely you'll get one that can't get much more than a few hundred mhz above stock speed before throwing up on you. It's good to learn, and a neat subject to study, but if anybody tells you massive, eyeball burning overclocks are cheap, effective, and hey, ANYBODY can buy this processor and get a massive speed boost is smoking something reallllllllly good, and they're neglecting to tell you much of the added cost that totally negates the savings on that cheaper proc. Plus, the majority of overclockers push to the limits, see how fast it can go, for how long, without crashing, but very, very few leave them at that speed. They'll back down to a moderate overclock and run them that way, rather than burn out that lucky piece in a year.
How far you can go.......if you don't have good cooling, you'll reach thermal shutdown very quickly, the processor will just stop. Likwise, even if you have top-notch cooling, the processor won't thermally overload, but it'll blue screen on you, as it can't handle the data stream you're throwing at it. It's pretty easy to spot......you'll crash. Hard. A lot. Go up, slowly, at the smallest increments, and stress test. Stop when you blue screen, or your temps start going crazy, and back off to a safe level. That's your max, and you won't get any farther. You can sometimes raise core voltages and memory voltages to stabilize things, but if you plan on doing this on a new i5/i7 Intel, do NOT go above 1.65v, you'll blow the memory controller, destroying the processor. Tiny, tiny increments, not huge jumps. Little baby steps is the key, don't start ratcheting things up in gigantic bumps.
All that said........it's an absolute thrill when you do get the right proc, the right hardware combination, and a little bit of luck, and crank a CPU well beyond expectation stably.
Faster than the speed of snot
Two wrongs don't make it right, but I sleep pretty good at night