Scyther used to be a serious OU threat, with the speed and power to deal monstrous amounts of damage to anything. It's ultimate goal was and is to be the perfect revenge-killer, dealing quick and powerful damage and never getting hurt.
The addition of Stealth Rock to the metagame ruined Scyther, crippling its strategy. Nothing since the introduction of Scizor in 2nd Gen has ever lowered Scyther's usage so badly.
I was outraged when Scizor took Scyther's place in the spotlight. 2nd Gen was built around improving everything with the new Steel-type, but in my eyes, unlike Magneton and Onix, Scyther needed no improvement.
In my personal experience, Scyther can strike out of the blue and devastate teams, but with bulky pokemon like Ferrothorn used so often, its capacity in OU is rather limited. Always pair Scyther with a Rapid-Spinner or a strategic hazard-avoider like Magic Bounce Espeon, and stick to UU or RU when bringing out Scyther. I always put Scyther on well built teams that have 5 great synergic pokemon. In the last stages of each battle, I send out Scyther like a secret weapon to rip apart the remains of the foe's team.
My favorite moveset uses Pursuit, Aerial Ace, Bug Bite, and Brick Break to make good use of Technician, but a moveset with Feint, Reversal, X-Scissor, and Endure that uses Swarm and tries to get to 1 HP isn't too bad. Like I said, Scyther is best in the lategame, so make sure that the opponent's priority-users are gone before using either of these movesets.
One last thing that may or may not have been said already: Never use Scyther with Eviolite. It's the most insane thing I've heard, and stems from the incorrect assumption that anything that can use Eviolite should. Scyther is way too fragile to worry about defenses, and Life Orb or Choice Band will help its strategy the most.
I'm a huge Scyther fan, and I hope that the higher popularity of the RU tier will restore some of its faded glory.
If you're creative, don't copy and paste this into your signature (Started by 3.14kachu).