How I Use It
I've used Thimble off and on for about a year in informal after school education. I've found more immediate success with X-Ray Goggles (also from Webmaker) and the few times I've started youth on Thimble, they've preferred other tools with fancier immediate outputs. In my own learning of HTML, Thimble has been invaluable (as written above). I started with code.org (and then onto Code Academy) and have found Thimble to be a useful place to apply some of those skills, but it's helped that I also have two of my own webpages. Other adults I've talked to have learned well from Thimble remixes but my own interest -- and most of the work I've done with youth -- has been starting from a blank page.
Thimble is part of a larger suite of three programs from Mozilla called Webmaker. It is undoubtedly the most powerful (at least, in terms of making a webpage) but the ease of use is really pretty minimal: features include a left and right window in which to code in raw HTML and immediately preview the results but without any "drag and drop" suggestions it's hard to know what HTML to use. Thimble does nicely color codes different tags and comments but it's a pretty long learning curve to produce the fancy CSS style webpages youth are used to... I would be surprised if any students under 13 or 14 really engaged with the tool and, while it has potential to really educate, I think many kids will be underwhelmed by it. Users with a lot of patience will really excel from Thimble's immediate feedback on weather HTML code is correct (as they will see it either work or not work on the right side of the screen) but pop-up suggestions about wrong code are a little cryptic. For my own learning, I found the Thimble in combination with Code Academy was a better way to learn simple HTML code, but I found that this knowledge was still a pretty far way from CSS. I've learned as well from changing values in remixed code and seeing where, in the code, those directions originate (in order and format) but, all in all, Thimble seems like a work in progress. Having worked a little with the Chicago Hive (a program of Mozilla, the folks behind Thimble), I know that they are looking at improving the tool.