It's great for a reason that won't necessarily recommend it to most habitual players of VNs: the characters are very finely drawn (I don't mean visually, though that's also true) and the dialogue between the siblings is incredible. I'm not sure I want to use the word "realistic," because honestly most people aren't as clever as the siblings, but they're the sort of people I befriend and enjoy talking to...and this is especially true if they're lovers...but even if they're not, they're the sort of people with whom I'll never, ever be bored. Haley in particular is as vivid and coherent a personality as I've ever seen in a VN. Here's a huge caveat, though: if English isn't one's first or secondarily fluent language, the brilliance of the dialogue may be quite easily lost.
The sex so far is nice enough, and the secondary NPCs are impressive in their own right, but even though the titular relationship remains almost entirely unfulfilled, I don't even care because they're so beautifully, insanely wonderful with each other.
It's definitely not a game for the impatient, though. There's sex pretty much all the way through, but it's not the sex at which the game hints.