Been hijacking it for years. Seems stable
Can only hold 2MB of data in Opera, much more in other browsers...
Is totally cross-domain though and should not be used for sensitive data.
But I agree with you: There are several more well defined possibilites for offlline storage (but as oftimes not very cross-browserish if you want to support IE). The best choice is a good library to mitigate browser differences like:
Also (off-topic) ran across this rather scary library that exposes just how many ways we can store persistant user data: