To The Stars is one of those precious fanworks where the author absolutely pours themselves into it. Bringing post-Madokami magical girls into a military-sci-fi 25th century that ranks rather high on the hardness scale is one thing, but the amount of worldbuilding Hieronym puts into the setting's cultural, psychological, sociological, governmental, and economic aspects is mind-boggling. People who like lots of detail may find this a huge draw.
The characters are also a strong point; some PMMM characters remain on the scene, but there are many magical and nonmagical characters to deal with as well. Ryouko herself is an engaging and sympathetic protagonist, subject to a feeling of listlessness many readers may identify with.
However, if the reader goes into TTS looking for the same stuff they saw in-show, they may not find it. The sheer textual mass of the still-in-progress fic might also turn away readers who prefer shorter works. However, those who enjoy an interesting and deeply-thought-out spin on magical girls (IN SPACE!) may find this to be just what they were looking for.