It's an interesting API, but currently only available in Chrome and I don't think standardisation has finished yet so it could change and break with new Chrome releases.
Also, it only works in a sandboxed file system - as if each website had its own empty drive. You can't just read and write files to disk like native apps can. Basically it's like unlimited WebStorage, which could be useful, but is a lot of work to really justify it right now.