Pretty damn good.
It's very difficult to find a fanfic where the bitching, the fanservice, the unresolved sexual tension, and the resolution thereof aren't just there to titillate the audience but really ARE part of the story. This is one of those fanfics. In Amplify, you get a real sense of intimacy in how Soul and Maka relate to each other as they go from friends to friends who very obviously want each other to lovers to seriously sizing each other up for marriage. I absolutely loved seeing the soul link evolve throughout the story. And yes, it really DOES have some of the best smut I've read in a fanfic. (Hey, I'm a woman. I can't help it.)
While this fic may begin like many other conventional Hurt/Comfort fics, it quickly proves itself to be far superior, exploring the different aspects of Soul and Maka's relationship: as friends, flat mates, battle partners and, eventually, lovers. This fic is beautifully written and incredibly engaging, with the rare added bonus of being pretty much spot on character wise. Later chapters get very steamy, but Marsh's gorgeous prose ensures that everything stays believable, and the story manages to avoid the many romantic clichés that often pop up in Soul/Maka stories. Read this, you will not regret it.