In Episode IV, the Tantive IV is taken by the Empire. The stormtroopers wrecked rebel booty. They weren't even aiming with the sights, unlike the rebels, and they easily killed their way through.
Later on, Ben and Luke stumble upon a raided Jawa camp. Ben takes a look at the wreckage and says, "These blast points; too accurate for Sand People. Only Imperial Stormtroopers are so precise." So we have a Jedi claiming they are good shots.
But then we get to the Star Destroyer. From that point on, they seem to suck. But why? Because they were ordered to. In the Star Destroyer, they were acting like a bunch of pansies. Aiming badly, and running away in bulk when Han Solo charges at them alone. And then, when Ben and Darth Vader start fighting, the stormtroopers guarding the Falcon WALK AWAY FROM THEIR ORIGINAL ORDERS to WATCH A FIGHT that they DON'T GET INVOLVED IN. In a scene right after the crew takes off in their ship, we hear Grand Moff Tarkin talking smack to Vader about USING THE FALCON to FIND THE REBEL BASE, saying he was taking a risk. Vader needed the crew alive so they could fly back to the rebels to be followed.

In Episode V, the stormtroopers are performing less than desired in Cloud City, as well. Well, at this point, Vader knows his son is a Jedi, and was using his friends as bait to lure him to Cloud City. Luke would feel his freinds' passing away if they were killed, plus, there's no use in a dead hostage. Vader plainly stated that he was only going to hand Han over to Boba Fett, and that he wouldn't kill anyone. Again, the Stormtroopers weren't allowed to kill anyone.

And now we come to Episode VI, the film everyone points to when claiming the Stormtroopers suck. "They were beaten by care bears with sticks, they must suck." The crew was caught by these care bears and were going to get eaten, they must suck. Because of the lack of technology that couldn't be used, the film wasn't able to get across the fact that Ewoks are a warrior species. They know their land, they know how to hunt, they know how to fight, they know how to kill. They, and the rebels, had camouflage, the stormtroopers had white suits. During this fight, we could see the stormtroopers using their sights to aim. I hardly think one would get WORSE at aiming when using his sights. Plus, stormtroopers were able to shoot R2-D2, an ewok-sized droid, and Leia. They were also attacked front and back, but still held off a while. The stormtroopers had no advantage on Endor, except the AT-STs, but they have proven not to work well in a forest. No camo, no cover, ambushed, no knowledge of the land, outnumbered. Nobody should be making fun of the stormtroopers' skills in combat for this, rather, perhaps their lack of being on guard.

So there you have it. Stormtroopers are good soldiers, so good, that they obey orders to act bad.

Don't Forget A stormtrooper hit their head
And Han Shot First