Rated 3 out of 5 stars
First, the bad: if you add a word like, say, AutoCAD to the dictionary, it won't accept the word "AutoCAD's", and you'll need to add that too. The number of word it has saved seems quite low (it doesn't accept the word "online" until you add it, for example), and I think the red squiggly line to highlight errors could be a bit easier on the eye.
That said, let the lack of errors in this message prove that it works. A multi-line text box is an online text-box that you write in that has more than one line. So the spell-checker doesn't work on boxes that ask for your name for example, but it does work on Youtube comments, and this box I'm typing in to write this review.
I'd say it's more useful to have than to not have. It checks spelling in emails too (works for Outlook and Google Mail at least, haven't tried others), and once you've added the words you use regularly, it's really pretty handy. It's just a bit of a pain to install fresh and add the 20 or so words you regularly use but the dictionary doesn't work, but that's just the lazy complaining.
Also works on Firefox 29.0.1, the one I'm using right now. Although it didn't just recognise the word "Firefox", so I had to add it. You'll have to do that with company names etc.