I love Spuffy because I love Buffy and so far Spike is the best partner she has had. Their story is epic and has everything, it's excitng, compelling, poignant, funny, violent, passionate, evolving and lasting. Oh, and hot, their chemistry is amazing.
For Buffy the mission is what's important, and she needs someone who will make her their mission and that's what Spike does. Within their relatinship they are equals, but he is happy to be her second in command and let her lead when it comes to the battle. Spike admires Buffy's skill and strength, he is proud of her and her achievements, there is no petty jealousy there, or bruised ego.
They are two sides of the same coin, dark and light, Vampire and Slayer, they just fit together. At the darkest times of their lives they turn to each other. They find comfort together. They constantly challenge each other to be better and stronger, they know the others faults, but are endlessley supportive. Like any couple who truly love each other, they are willing to change and compromise, but always be honest (sometimes brutally) with what they need from the other.
I cannot imagine either of them being as happy or fulfilled with anyone else now.