I use it at one of my master courses since 2019 or so.
Great book, the students can follow it just fine and workout at home the exercises and particular examples from it.
Happy to see it here this morning :)
Yeah, as a student I was taught Nielson & Nielson in a course organized by the authors, more than a decade ago. Most students struggled a lot due to its emphasis on abstract algebra.
As a result, the authors eventually published a much more friendly textbook: https://arxiv.org/abs/2012.10086. This one from Møller & Schwartzbach is also pretty great.