This is a slice-of-life game, heavily influenced by lad-lit novelist Nick Hornby, that deals directly with a romantic relationship and its breakup. That's a pretty rare thing in IF, and Mix Tape gives some hints as to why.
The author's focus is definitely more on the writer side. Essentially all of the significant actions come pre-scripted; only one scene involves much interaction, and even there your actions are about preparing dinner, rather than directly engaging with the relationship. Much of the significant plot is doled out in walls-o'-text. The prose wanders, a good deal of the time, into overwritten or overwrought territory; it avoids being bland, it maintains voice, but it's in need of a ruthless edit and some repurposing to fit its medium better. Significant action is pretty limited throughout; veering from the script will either get you stuck entirely, or dragged back on course.
The central relationship concerns Peter and Valentine, twentysomethings from nowhere in particular. Although you play as Valentine, the protagonist of this story is definitely Peter; Peter is the one whose interests dominate the narrative. Val isn't given much that makes her stick in the mind as an individual. In the frame-story, wherein Peter gets Val to burn her scrapbook of their relationship, it felt to me very much as if Peter was using Val as a prop in his own internal drama while justifying it as a necessary step for us.
A lot of the problems with characterisation are ultimately interaction problems: Peter isn't deeply-implemented, so he comes off as distant and inattentive. Peter's role is to direct the plot, so he comes off as controlling. The combination makes him feel self-absorbed and emotionally manipulative. (True, this is meant to be about a failed romance, not a healthy one. But it's meant to resolve into a failed romance but a reaffirmed friendship, and doesn't really succeed at it.) But it's also a problem with the writing: Valentine mostly thinks about Peter, Peter mostly thinks about music. It's hard to form much of a picture of Valentine other than as Peter's cute girlfriend.
In this particular corner of cultural history, we tend to have some fairly strong feelings about parity in romantic relationships. But the basic formula of interactive media involves a highly asymmetric relationship. This means that IF stories about romance are a very fine balancing act; small errors can have far-reaching, unintentional overtones about manipulation, coercion, emotional blackmail. Moments intended to be touching often become creepy even in more traditional media; IF is even more vulnerable to this. (Violet, a more mature piece than Mix Tape, still ends up falling into this trap.)
The game's structural conceit is a retrospective of a romantic relationship in the form of a selection of songs. This is kind of self-indulgent on the face of it, and suffers badly because the songs aren't (and can't be) included in the game. This was perhaps more of an issue in 2005 than it is now, when more or less any song can be listened to on YouTube; but it still has a translation problem if you're unfamiliar with the songs, dislike them, or simply have no emotional resonance with them. (For me, most of it falls into the category of Earnest Indie-Pop Rock with a side of Boring Indie That Nerds 5-10 Years My Senior Like.)
(I liked this considerably more when it first came out, but on re-examination its flaws feel far more acute. The score here splits the difference.)