Hmm, this is an interesting thread, and I've been itching to discuss Spike, so this'll be a long one.
The confusion on Spike's humanity can either be blamed on Joss's failure to maintain consistent canon, or can be considered just another interesting facet of the Buffyverse. I personally believe the latter, but one way or another, Spike was always different.
The emotions he showed were very genuine, very human, and based on countless episodes, there is no denying that he felt them. The examples of this are so numerous that I'm having trouble pinpointing just one at the moment. If you start from the beginning, there's the fact that his love for his mother stayed with him after his turning, enough that he wanted to "save her" and bring her with Drusilla and him. The next example is Drusilla herself; he most certainly loved her. It may have been twisted and demonic, but it was real nonetheless.
I do believe his love for Buffy started as an obsession, and I think he loved her that way long before the chip (Drusilla nearly confirmed as much with, "I can see her, floating around you"¦ why won't you push her away?"). But the last few episodes of Season 5 showed the change in him; he didn't give Dawn up to Glory despite the torture, he grabbed a sword with his bare hands to save Buffy, and he continually marched into battle knowing he could very well die"”all of that was selfless. It might not fit with the canon view of vampires, but it was there nonetheless. This point is further shoved home with his complete breakdown upon laying eyes on her body, and with his subsequent behavior. How many evil demons would stick around to baby-sit the younger sister of the Slayer, just because of a promise and a love for said Slayer?
On a different note, I think there's a reason for the way he killed while he was an unhindered, chipless vamp. While it can't be denied that he became a demon, and was therefore different than William, I think William's experiences played into Spike's behavior. He was treated like scum most of his life, denied by the woman he loved because he was "beneath her," and then he was given this extraordinary power. He no longer had to respect the laws of men, or even place value in human lives. He went from being below everyone, being William the Bloody Awful Poet, to being powerful beyond imagining. The way he altered his accent expressed his need to act tough, to create an image other than the one he'd always worn. Also, I think he found in himself a fierce love for violence and fighting ("When's the last time you unleashed it"¦. nothing but fists and fangs?").
His history aside, the real reason he was able to change, I think, was because of the chip. This was a situation completely unique to Spike; no vampire in the Buffyverse had ever had their power to kill taken away from them. It forced the parts of him that weren't the demon"”the parts that still managed to maintain his humanity"”to surface. What would anyone do when their purpose in life is taken away from them? Find something else worth living (er"¦ unliving) for. It was either bother Buffy and the Scoobies or be alone, and I don't think Spike could have handled the loneliness. So he fell in love with Buffy, bonded with Dawn, realized that Willow was kind even to a creature who'd previously tried to kill her, and suddenly he had all of these ties that he'd never meant to make.
There was proof of his change when he tried to attack the girl in the alley in Dead Things. He thought he could hurt humans again, so of course he went out to find a victim. Once he'd cornered her, however, he had to talk himself into it, force himself to think about Buffy and her cruel words and the fact that he was a monster. But the pleasure of the kill was gone; the lust for violence (at least in this way) was gone. The only things that allowed him to continue with the attack (and I do believe he would have killed her if the chip hadn't worked) were his bloodlust and his need to believe that he hadn't changed.
In Season 6 it became murkier, and there were parts with which I disagreed. Without delving into the twisted relationship between Spike and Buffy (and it was twisted"”on both sides), I think the fact that he got a soul is solid evidence both that he loved Buffy and that he was different than almost any other vampire. Even Angel, who was the reigning Vampire Hero, didn't choose to have a soul. Spike's decision to regain his, his fight to win it, showed that he had indeed changed. No other "evil, soulless thing" would have bothered, because no other vampire could have loved Buffy the way he did.
That leads to an important question: could Spike have found his direction without the soul? I think so, yes. Obviously that is just my opinion, but I think there is enough evidence to say that Spike is, indeed, an exceptional vampire.
So, I'll end on another crucial question: Did Joss and Co. mean to do it? Was Spike an accidental break from canon, or was there purpose behind his character? Well, consider Buffy, herself. When has she ever fit the mold of the Slayer? When has she meekly taken orders from the Council, or cut everyone out of her life to do her duty, or even stayed dead, for that matter? Buffy The Vampire Slayer constantly showed things differently than they appeared. It was always about presenting us with something, building up our expectations, and then smashing them down around us. The show was never afraid to dive into the gray area and shake up the rules. I think it's entirely plausable that Spike was one of those characters intended to break up the black-and-white views of right and wrong.