I wanted to wait to answer this until I was home at a computer so I could really write this out. I had a lot of questions about compression gear before I bought any, and I did some research to figure out if it was actually worthwhile.
There’s a lot of arguments that compression socks don’t do much for the “regular athlete,” you don’t see many elite runners sprinting with compression socks on (did you see any at the olympics?). Originally compression socks were made for diabetics looking to improve lower leg blood circulation, as well as treat other various symptoms such as varicose veins, phlebitis, edema and deep vein thrombosis. The thought is that the tight knee-socks provide upward blood flow, refreshing deoxygenated blood that moves rather slowly through your veins. This seems reasonable: venous blood moves when skeletal muscle squeezes and pushes the blood along, it could feasibly be helped with outside pressure providing extra force.
While short distance runners are rarely seen with compression socks, it seems that endurance athletes may benefit most from compression gear. Beyond increasing blood flow (and therefore lactic acid clearance, the “burn” of a workout), compression socks are believed to increase muscular endurance by reducing the vibration felt with every footfall. Through reducing the unnecessary vibrational motion of the muscle, you slightly increase efficiency and therefore leg power. Whether or not this is actually beneficial to the runner is more of an individual issue: the event, the type of sock, and the usage can vary greatly. Mostly these socks are used for recovery after hard runs, but other athletes such as long and triple jumpers often use compression socks to decrease muscle vibration and increase balance and proprioception. This increases their leg power and therefore jump distance.
In research studies these claims have been substantiated, showing that compression gear usage resulted in lower 5K times and improved running economy when worn during a run. How far these results go is questionable, and definitely depends on the athlete. It’s suggested that compression gear will be best for the endurance athlete who will benefit from minimized long-term muscle fatigue, as shown by the popularity of the socks among distance runners like Lornah Kiplagat and Paula Radcliffe.
The bottom line is that you should wear them if you want to, but be willing to shell out extra bucks. They’ll be more helpful the farther you run, or for wearing as post-workout recovery. Personally, I find that they’re somewhat helpful. Extra thigh compression has helped me get through long runs where my quads were nearly shot, so I’m a fan. I wouldn’t say that they are a necessary item for top performance, but they do provide some measured benefits.