Question: Do you think hackthissite.org is a good tool to master web hacking?
Answer: Thanks to Frank Smith
The missions on that website are pretty realistic in terms of real world hacking. But if you finish all the missions you will have only scratched the surface of "hacking". There are literally dozens** of site that will provide you similar missions but they all lack one thing, fundamentals. Finishing all the challenges is great but it won't teach you ABCs of security. If you really want to understand Web Application security I recommend Web Application Hackers Handbook. This books is the single best source of learning security for Web Applications.
Since this book only focuses on WebApplications, I would recommend Counter Hack Reloaded as a good counterpart. Its a bit dated (2006) but it goes in great details about fundamentals and covers a huge ground.
**List of sites similar to Hackthissite.org