It's pretty easy to jailbreak (at least the older devices like ours) and there aren't any real downsides.
What I like:
screensavers hack -- my Kindle is NOT special offers, so I would rather the screensaver be the cover of my currently-open book.
You don't need LibrarianSync/CM, your Kindle can have collections managed via calibre even without a jailbreak.
Font hack -- some people actually care about fonts, I don't. If you do, this gives you control over the fonts for your book.