I joined the Stormcloaks and done a few quests for them liberating whiterun and Markath so far but they're coming across as a bunch of bad-eggs so i want to leave them and join the imperials instead. So is it possible to leave the Stormcloaks and join the imperials?

Nope, once you join one side, you can't join the other.

Crud, I'll just have to settle for punching Ulfric in the face, it'll be worth the bounty.

thats just to give a grey feeling to the other wise cut n shut fact the imperium is run by the thalmor wnd they are a bunch of bastards so they made most of the storm cloaks ppl tossers and put in npc dialogue for them being smelly uncivilised brutes, yeah im loving the cliches... id still stick with what you have, its better than helping the aldermeri dominion and forsaking talos, remember its a rp so just make your own interventions, when the wars over just kill the ppl you dont like and give skyrim back to the ppl.