Pros: (1) self-paced, (2) context-based, (3) context-driven, (4) presence of immediate feedback and correction. For example, one can complete one section for a week to build a strong foundation of skills set. Further, work sets are based on real-world problems, such as building a search engine and a web crawler. Finally, video feedback for solutions always provide corrections at the end of problem sets.

Cons: (1) limited opportunities for discussion (as with most MOOC platforms), for example, it is possible to consult mentor and peers in a classroom or workshop and discourse, but in MOOC even forums have delayed feedback.

There's a lot of positives about the Udacity front-end dev program. They make an effort to build community and provide assistance. There's always someone who is willing to help you. There's a lot of quizzes and projects so you can practice coding. The price is pretty reasonable and you can always go at your own pace.
The cons? The lecture videos aren't always very clear or easy to understand. It takes a lot of motivation to get through the program. I'd recommend doing the program while participating in a local meet up with more experience developers to help tutor you when you get stuck.

I love Udacity. I watch many free online courses (Intro to inferal stat, Intro to data science, python basic) before I made a decision to apply for Introduction to programming nanodegree.
They offer 7 days trial. I was worried before I enrolled because there are some reviews that they give back feedback late. Once I tried myself, the feedback is back within 2 days.
Personally I don't need 1-on-1 coaching. I can follow the lesson without problem but I valued for their feedback. My projects are checked thoroughly. Without the feedback, my code is good but not great. And if you can finish the course within 12 months, it will cost only 100 USD/month. Well, 100 USD/5 projects and they will check until it passed all the criteria. Moreover, the course is not too easy to get me bored.
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Support is very limited. Don't think that it's right for an absolute beginner that need substantial feedback. I would recommend to watch the free videos and keep your money for it's not worth it. The money that you pay are for support are on projects only and not the pre-courses that you will need to review that also have mini projects that you will need to complete to help you learn. If you feel that you need a lot of support, Udacity is not it. It's best to go to a full blown boot-camp type of school.

Was ok