This will be really nice for WordPress development, since you bork the whole backend whenever you break some of the core files, such as functions.php (which can be done simply by forgetting a semicolon). Using this should be much easier than logging into cPanel and much more portable than using FTP. Cool beans.
I like all the progress I've seen in web based editors over the years. However, one feature I use a lot in my IDE but have not seen in any web-based editor is code completion. When that gets implemented well, then it will be hard to resist using something this.