I think that people's opinions of Snape are based largely on whether or not he is a "good" person. This is kind of a pointless debate because no character or human being is purely good or purely evil (Sirius mentions this in OotP).
Snape's circumstances didn't exactly give him the best chances in life--growing up in an abusive home, then finally escaping to Hogwarts only to be bullied there as well. When he joined the Death Eaters, I don't believe that he did so out of a desire to hurt Muggles or Muggle-borns. He had a fascination with the Dark Arts (as many people do with the forbidden or taboo) and an overwhelming desire to be accepted. People like Lucius Malfoy made him feel important. They valued his skills and didn't reject him because he had poor hygiene and stunted social skills.
As a Slytherin, he did put his own well-being first (at least when he was young), and he was willing to let others suffer if it meant that he was being valued and included. His ability to love someone enough that that love overpowered his own desires was, as Dumbledore knew, his most redeeming quality.
Of course his treatment of Harry was inappropriate--no one can argue against that. However, consider that every day he had to see the physical proof that Lily chose James over him. Seeing her eyes in his face couldn't have been easy. And Harry did have a penchant for getting into trouble, which Snape obviously saw as intentional. Despite this, Snape did save him repeatedly, often from Harry's own carelessness (or admittedly blundering heroics, which Slytherins see as carelessness in disguise), or from Dumbledore's questionable habit of letting Harry test his own capabilities. Snape put himself in considerable danger to do so, as well as to further the Order's cause.
Snape isn't a "nice" man, or even a completely "good" man. But his actions (to me) speak more loudly than his unpleasant attitude. If you read his scenes very closely, you'll often find that he had more reasons than bitterness to act angry or unfair (often fear--of failing to protect Harry; of failing to conceal his own inadequacies; of revealing his one 'weakness,' his capacity for love). Sure, sometimes he's just a petty dictator, but many people act unfairly based on unreasonable grudges, usually because they want to hide the pain they feel at past humiliation. The bullied often become bullies to compensate for their former lack of dignity.
I could go on for ages about this character (by far my favorite), but I'll end by saying that Snape was a brilliant, sarcastically amusing, damaged person locked into a life of servitude and repentance because of a horrible choice that he made when he was a teenager. He's bitter, petty, and indescribably sad. He's an anti-hero, and in my opinion he's among the most complex characters in the series.
