Rated 5 out of 5 stars
There's a lot of complaining in these reviews, but pretty everything these people are saying is untrue or just plain ignorant.
"Weak privacy" ---- Diigo's "private as default" setting is enabled by default upon installation. Yes, it's browser-specific and it would certainly be best if the setting was stored online, but still, it's a user's responsibility to check the privacy settings when they use public computers to do things they intend to keep private.
"Can't disable public notes" ---- Uhhhh, yes, you can. In fact, there's multiple levels of Public, Private and Group visibility of notes and they're right there in the pain Diigo menu on the toolbar.
"Doesn't work" ---- I've been using Diigo on Firefox 3.5.9 and 3.6.3 for the last 3-6 months and have never had any problems. However, I did email Diigo about a feature request and received a response within a few days.
Anyway, I do have to say that the varieties of highlighting that Diigo is capable of are a bit limited. If you want a really robust highlighting util for your browser, then I recommend checking out Wired-Marker. On the other hand, the Diigo's social networking capacity gives it an edge if that's the kind of thing your into and has the potential to be one of the web's "next big things" if it's developed properly.