Well, it mostly depends on how much time and effort you want to spend, and how much you like to tinker.

You can potentially get between a 0 and 20% speed boost overall I'd estimate, but the 20% boost would be very rare IMO, but for most folks a 10% boost should be do-able for not much extra cost.
If you look at it from a $ perspective, OCing probably comes out close to even depending on where you are buying on the speed curve. For example, if I spend $50 more on fans, a killer heatsink, and some higher quality parts for my rig so I can OC it properly and still have a stable rig, I have to see if that $50 could have simply bought me a faster processor. However, if I'm buying closer to the top end of the price/performance range, the next CPU up might be several hundred dollars, so that extra $50 on good cooling and parts might net me more benefit than I could gain any other way. Also sometimes I won't OC right away, but after my system has aged a bit and needs a bit more power, I'll OC to extend it's life. This is only normally possible if I chose the right parts in the first place to be able to get a stable overclock, but personally I choose parts that are OC friendly so I can get extra life from my gear. Depending on how much work it is (and how much time I have), on some stuff I'll tweak and OC it from day 1.
But for me, it mostly boils down to wether or not you will enjoy tinkering with your rig and tweaking it. OC'ing will certainly take some time between researching parts, purchasing and installing some gear yourself, and actually adjusting your system and testing it. If you want to learn more about computers in general and you would enjoy spending the extra time doing this sort of thing, it's definately worth it. If you are busy and don't have the interest or time to screw around with your PC, then just buy the best you can afford and be done with it.
So only you can make the decision to OC or not, but I hope I helped shed some light on it.
