I am not sure I'm adding anything new to the insights above, but I just wanted to add my vote. I actually love Robin and Faith together. I see Robin having a lot of insight and the patience to break down Faith's walls and 'surprise' her.
Faith thinks she's seen everything (but she's wrong). She thinks all men are dogs. She doesn't think any man is worth the trouble of getting seriously involved. She's afraid of involvement or anything that might require trust.
Robin is someone who is confident enough and a man of the world enough to not bore her -- and to prove to her that she has lots more to see and learn. I always think he's more 'worldly' than she is... educated and experienced by virtue of being exposed to a lot of life -- as well as knowing first hand about the slayer. He's not afraid of her or her strength. He is also gentle and I think Faith has not had many men be gentle.
I also loved their chemistry.
I am not really bothered that he dated Buffy first. It actually is telling that he has a type... he likes strong women. But Buffy isn't for him for a variety of reasons, including simply not being interested in him beyond finding him initially attractive. The suspicion that grows up very early in their romantic interaction also pushes them back into their own corners. I don't think their chemistry is anything as good as the chemistry between Robin and Faith... Robin and Faith make me laugh, touch me and intrigue me. I find them sexy together. I got the feeling Buffy was going through the motions even though I think there was some attraction, it wasn't all that. Their personalities (Buffy and Robin) kind of clash.
By contrast, Faith and Robin just wanted a bit of warm comfort, so they connected without too many expectations -- but I think from the beginning, Robin was intrigued. And Faith was surprised to connect... and it's clear she has.. because she's running away, exactly the reaction of someone who doesn't trust easily when they find themselves getting attached, However, as soon as Robin is injured, you see her real show concern for him.
Just because they are an interesting match doesn't mean I don't foresee problems.
She has little formal education. (canon is that she dropped out of high school). Robin has to have a master's or maybe even a PhD in order to be a principal. In addition, he's been raised (and probably educated by a watcher. I suspect he learned a lot of about being a watcher and about slayers) I think this knowledge deficit would bother Faith more than it bothers Robin. I think he can see she's smart and that she has potential for miles, though I suspect he might think about it, I suspect his answer would be to encourage her to develop herself. I think she might meet some of his university friends and feel intimidated and wonder if he'll stay with her when he has his pick of smart, attractive, educated, high earning women, and also maybe even black women, though there's no sign he has any issues with dating a white woman, the sample size in Sunnydale wasn't exactly big.
I've shipped them as a secondary ship in at least a couple of my stories and even started a side story from the main story that looks more deeply at their relationship. I love exploring them.
I definitely think they're my 'one true pairing'(for Faith) in the Buffyverse... In crossoverville, there's a lot out there. (I like writing crossover stories)