My experience
I did 30 Days of Web3, because I liked the concept of spending a month on a program, which requires less than 2 hours per day, and have a good overview of what you need to know to make a dApp by the end of it. Interestingly enough, it's actually not made to be done in 30 days, there are regular milestones through the curriculum, and it ends on day 16... I guess you could say that it takes more than 16 days since there's still a chapter left after that, and the rest was probably added to give people a bit of margin to finish in time.
I was pretty happy with the structure of the course at the beginning: a simple article to give some basic concepts and some additional links if you want to dig deeper by yourself. The rest felt a bit less structured. The gas cost is mentioned once, and you can see, here and there, some basic explanation of why the project and code are made a certain way, but it feels like the course authors didn't make a list of which concepts were important to explain thoroughly. Rather, it felt like the project was made first and the explanations were added afterward.
On the plus side, it requires you to use tools that you would need during development (Remix, coding with Visual Code Studio, looking up information on the Polygon Scan...). Which is good to give you an idea of what you would need to do to build a real project, and which resources you can use.
It also has a Discord channel where you can ask for help if you get stuck, which I am guessing can be very helpful for some people. Especially since some of the code that they provided was not working when I went through it (which is fair, since it is because of some breaking changes introduced by a dependency).
Overall, I feel like you could get through the course without retaining much since it involves a lot of copy-pasting and it's not very thorough in explaining some important concepts. I think someone coming from web2 would know pretty fast how to make a web3, but might produce low-quality code if they don't do their homework to learn further.
What you will learn
Web3 + Blockchain + Smart Contracts basic concepts
How to set up a Wallet
Remix
Solidity
Polygon
Hardhat
The Graph
Rainbowkit
Web3Storage
Ethersjs
Positive
✅A good overview of which tools you would commonly use to create a dApp
✅Gives a better understanding of how a dApp works by walking you through one
✅It gives you the option to further learn by yourself by customizing the dApp further
✅It gives you access to (a small) online community
✅Possibility of getting help through Discord
✅Quizzes to test your knowledge
✅Possibility to give feedback on the course (and for it to improve)
Negative
❌Might be confusing for beginners (especially if you didn't work in web2 before)
❌It's possible to go through the program mostly by copy-pasting
❌Some of the code doesn't work (is out of date)
❌It doesn't emphasize important concepts (security, privacy, gas prices...)
❌The quizzes are really basic
Further resources
If you want to see more resources to learn about web3, this page gives a great overview of them.