The stormtroopers have changed a lot. Much of it due to the conflict between what the movies tell us about them (*Only imperial stormtroopers are that accurate" - Old man wan) and when we see (miss, miss, miss, miss).
My favourite theory is one I heard not long ago, and it centers around Darth Vader and his feeling toward his son. The main part is that whenever Luke is nearby Vader will use the force to mess with the stormtrooper's aim and cause them to miss. When he's not around, or busy fighting his son or the emperor or his own emotions and therefore is too occupied to do so, the stormtroopers are elite again.

So on the ship where Leia is captured they wipe the floor with the rebels.
On Tatooine, they are scary and professional.
On the deathstar they suck.
...
On cloud city things go south.
...
On Yavin 4 Han and Leia are surrounded and caught with relative ease...

It's not perfect, but I like it.

Anyway, the most important part of the imperial army is that it is heavily influenced by the old Roman empire. The stormtroopers (different kinds of legionaries) are EXTREMELY well-trained, but has only received their training under a very short time, most of the "training" is field-practice, ie slaughtering barbarians.