Tobar said:

The most important thing people forget is that Vader wanted them to escape. So the Stormtroopers were under orders to make a show of trying to capture them but allowing them ultimately to get away so that they can be tracked back to the Rebel Base.

Stormtroopers are no joke, we're told what crack shots they are and are given evidence of such. They were able to disable an enormous Sandcrawler with a few precisely aimed shots. And lets not forget what happened to Uncle Own and Aunt Beru.

In ESB they completely decimate the Hoth base. A base considerably better protected than the older Yavin one.

I suppose one could say that the only film which really depicts the stormtroopers in an incompetant light is ROTJ (which really isn't all the surprising).