Some SPOILERS if you've not finished SoS (I've not finished AFFC)...
I love Littlefinger, he's my favorite character. He's truly fascinating. He's easily the best player of the game in the series and I love his constant joking and cynicism and unapologetic selfishness.
But I think beneath that, there's more going on with Littlefinger than it would seem at first. There's the scene of an upstart not-quite-noble being publicly humiliated before the woman he loves (hopelessly because of his station in life, thus driving his all-consuming ambition). Then there's his clearly petty and insecure bragging about "bedding" Cat (true or not? I'm not done with AFFC yet) and the conflicting desire to both be Cat's savior and win her approval as well as get back at her and her family for jilting him. There's the scene of him playing in the snow with Sansa... careful manipulation or the childishness of a deeply unhappy man yearning for the simple joys of the youth he shared with the Tullys? And he did get Sansa away from the Lannisters. He had his ulterior motives, but it's tough to argue that she's worse off for it.
He is the classic Machiavellean lord, the only man in the series on par with Tywin Lannister. You may love him or hate him for it, but it certainly makes him fascinating. And while he's always looking out for number one, he's never sadistic or deceitful about that.
I've no clue what the future will hold for him, though as someone above said, in some senses he and Sansa are perfect for each other (kinky wish fulfillment aside). Nonetheless, I suspect Littlefinger will overplay his hand one day and suffer the consequences, likely at Sansa's hand for the betrayal of her father.
However, he could also end up getting the prestigious lordship he wants. I can't help but think that it was Littlefinger that secretly met with the Tyrells and brought them to King's Landing to support Joff (after the battle was over of course), and it was a Tyrell that later removed that king and caused the events that allowed Littlefinger to take Sansa out of King's Landing and gain access to the Eyrie. That sort of thing is rarely a coincidence where Petyr Baelish is concerned!