1.It's a fun hobby
2.Gain a bit of extra performance in certain scenarios
3.Benchmarks look much better.
1.More Stress & Heat (Better cooling solution/ventilation a must)
2.Wear & Tear (parts wont last as long)
3.Majority of all common tasks will see absolutly no gain. (such as web browsing, watching videos, listening to music, Email etc)
4.Higher quality parts will be needed. (More expensive RAM, MOBO etc) for best results.
5. Lots of trial and error to find that "sweet spot" with the potential of ruining Hardware or frying RAM/CPU
Just the sad truth about it.
For the record, I am a overclocker.