So I think for expanding your knowledge base while being employed or doing other things, it's quite good (and, there is also Udacity's MS program with Georgia Tech for more serious education and credentials). It is even perhaps worth paying the $100 a month yourself if you put in the work to finish a bit early (the deadlines are set so it takes 13 months if you don't try to finish early). But, there is a lot they could do to improve - the nano degrees really are a bunch of totally separate MOOC classes strung together.
The courses did not push me to work that hard - all of them had just one deadline which was the project submission, and there were usually 2-3 months between deadlines and I got away with only seriously working on it the last 3-4 weeks most of the time. That being said, the projects were of good quality - pretty substantial, with good rubrics, and good ability to choose from easy options or do your own things. And, once again my extensive background in CS and ML probably made things easier than it would be for many people.
To be clear, I did not mind the courses not being that deep since I did not really want to be really busy with courses while working full time. I just think if these are for getting into being a professional in the field, they are not quite as deep as I might like for that.
In comparison, I have taken classes in my grad school that I spent $1200+ on that I ended with anything to show for it.
I will say though that Udacity show more preview content and maybe a longer trial period (currently 7 days). I did try the Android nanodegree and felt it was not worth my time but that was because of my experience in it.