Why do so many people love Spike so much?
In no particular order:
1) He is a great character.
2) He is funny.
3) He is charismatic.
4) He is sexy.
5) James Marsters was amazing in the role.
6) He is a punk rock vampire! What's not to love?
7) He likes Ramones, Sex Pistols and Dead Kennedys
8) Billy Idol stole his look

9) He was the first non-conventional Buffy villain, with individuality, charm, humor, sex appeal and ambiguity that was world away from the Master and his minions.
10) He's a rebel - he didn't care about the vampire traditions and hierarchy. In his own words: "From now on, we're gonna have a lot less ritual, and a lot more fun around here!"
11) He got rid of the Annoying One.
12) Even without a soul, he was able to love strongly and be loyal to people he cares about.
13) He was complex and full of contradictions from the start, which made him interesting: already in his first episode ("School Hard"), we see him both as dangerous, predatory and violent, and as a devoted lover to Drusilla.
14) As a human, he used to be a total dork and a bloody awful poet.
15) Beneath his cool tough persona, he is vulnerable and romantic.
16) His ideal romance includes a lot of hot, rough, kinky sex.

17) He is brutally honest and doesn't mince words.
18) Although anti-intellectual, he is very smart and extremely perceptive (as long as it's not something that concerns his own relationships).
19) Unlike every other guy in the Buffyverse, he keeps his jerkass tendencies on the surface, and his softer and sweeter nature hidden underneath. He sure can be a jerk, but it's in your face and out there in the open, which is really refreshing after all those guys who seem so sensitive and nice, or heroic and wonderful, before their jerkass tendencies come to light. (Parker is the most obvious example, but Angel is a lot like that, too, and Riley is as well. And remember Ben? God save me from 'nice guys' who aren't really nice.)
20) His accent and use of British slang is cool.
21) While he may often be a jerk, at least he isn't paternalistic like Angel or Giles.
22) Unlike Angel and especially unlike Riley, he has great chemistry with Buffy.
23) He has great chemistry with pretty much everyone and everything. Even the crypt door and thin air (literally; see "Dead Things" and "Gone").
24) He doesn't have problems with accepting a woman's leadership in battle and doesn't worry that it will somehow endanger his masculinity.

But he can step in to take charge of the situation when she isn't able to (see: the Caleb fight in "Dirty Girls")
25) He understands that Buffy is strong and can fight for herself, not some little girl who needs protecting.
26) He had an amazing character development and a great story arc.
27) He is the only vampire who actually
decided to get his soul back and
fought for it.
28) He is not obsessed with destiny and the Powers That Be. He thinks that if you want things to change, you should make them change.
29) His relationship with Buffy made for one of the most complicated, risky, explosive and amazing 'ships on TV - it could be dark and disturbing to moving and beautiful. It was a lot more raw and real and deeper than the Bangel (not so) 'ideal romance' with its surface romantic rhetoric.
30) The show never tried to separate his good and evil side into a Jekyll and Hide dichotomy and call them by different names.
31) You never knew if he was going to be bad or good, he could go from incredibly sweet one moment to a total jerk the next or the other way round (not just in the same episode, but often in the same scene), from funny to deadly serious in a heartbeat, from hateful to lovable and from pathetic to heroic - which made the character all the more interesting.
32) He had a very sweet big brother/little sister relationship with Dawn.
33) He got along with Joyce even when he was evil.
34) He can sing and has a beautiful voice.
35) He makes fun of Angel's 'brooding hero' persona.
36) He is passionate and wears his heart on his sleeve
37) He likes to fight, whatever the odds, and is brave, even reckless - with his life and his body, just as he is with his heart
38) He actually died to save the world out of his own choice, which puts him in a very exclusive club (which currently, in Buffyverse, consists of Buffy and him)
...I could probably go on...
Just because he has a soul now doesn't mean he can't still be a bastard sometimes.
Of course he can! He would be boring and unconvincing as a character if he couldn't. Most humans can be bastards, and they have a soul!
Vampires with a soul can, too - see: Angel in S2 of AtS, in S3, in S5, in S5... (Locking the lawyers in to get killed, firing his friends from Angel Investigations, trying to lose his soul on purpose, using Darla to try to lose his soul and kicking her out in the morning, trying to murder Wesley, mind-wiping his son and his friends, being a petty jealous jerk towards Spike for getting the soul and becoming a champion and therefore taking his glory [because only Angel is the designated hero, according to himself!], killing an innocent man like Drogyn as collateral damage in his plan, using Lindsay and then sending Lorne to execute him when his usefulness was over, bringing hell to LA... And let's not get into what he did in season 8 of
Buffy...) And it's really those moments when Angel is dark or a jerk and does morally questionable things even with a soul, that make him an interesting character. (Well, except in S8. That makes him look like an idiot.)
As for Spike not being nice to Robin... it's hard to be nice and sympathetic to someone who's been trying to kill you. I guess it would make him a bigger man if he was able to, but I like it better when the characters act more... human, rather than like an ideal of perfection that you get to see in some movies and shows. (I felt the same about Angel not being able to forgive Wesley and trying to kill him in the hospital. Which was actually much worse since it was attempted murder, but I don't see people bring it up against Angel often.)
The thing that people bring up often is his attempted rape of Buffy. Yeah, that was a dark indefensible moment. The thing that people fail to mention, though, is that Spike himself knew that. He was so disgusted with himself afterward that he left, and went through some seriously painful trials in order to regain his soul, and ensure that he would not do something so terrible again.
What I find funny is people who say that they couldn't like Spike anymore after that... But let's be real, Spike had done much, much worse in the past. We just didn't get to
see it, and it didn't concern any of the main characters we care about. As soulless vampires, both Spike and Angel ("Angelus") had killed a lot of people, and committed a lot of metaphorical rapes (vampire biting and blood-drinking was always clearly a sexual metaphor) and I'm sure also literal rapes. Spike pretty much says it in season 7, Angelus hinted he did it to Holtz's wife, and he threatened Fred with rape in S4. But apparently people had no problems with liking Spike in S2 as a cool villain - when he was killing people for fun and blood - or in season 3 and 4 as a comic relief, when he was regularly making casual funny remarks about his murders? See also the funny but actually quite disturbing scene of Spike's attack on Willow in S4 "The Initiative" (the funny exchange in which they keep talking about his inability to bite her in the way people would talk about impotence, which underscores the impression that the scene is a metaphorical rape attempt).
It took for Spike to be promoted into a love interest for Buffy for the show to start treating his crimes seriously and ask some questions about his nature (Is he still evil? Could he be good? Can he really change?).
I liked him just fine until he got Buffy-whipped and then he was hit and miss for me.
Whereas being Dru-whipped was cool?
