A long and winding road to disappointment Rated 1 out of 5 stars
I imagine I am one of the longest users of Diigo around, given that I discovered it very soon after it came onto AMO. Unfortunately for me, this has led to a discovery about problems with the Diigo features that only crop up after significant usage over an extended period of time.
At the outset, Diigo was for the most part what I wanted in a research addon. It allows the user to select and highlight text on webpages directly and the text remains highlighted every time the user returns to the page. One can also add "sticky notes" to highlighted pages that pop up and display one's comments when moused over. In addition, before I discovered the amazing extension Autocopy, which far surpasses Diigo's functionality in this area while maintaining fantastically low overhead, I frequently used Diigo on new builds for Firefox just to be able to copy text automatically when selected. Actually, in Diigo's case, there is a menu that pops up if this feature is selected that allows you to choose to copy or highlight the text. These functionalities kept me using Diigo for quite a long period of time (two or more years), but ultimately did carry enough weight to either counterbalance the flaws with Diigo or beat out their competition.
* Not enough users: Diigo has attempted to become a social bookmarking utility of sorts, allowing users to make their individual sticky notes public (viewable by others) on a sticky by sticky basis. Unfortunately, in all my experience (and I had this installed and active for a couple years) public comments are extremely rare and you almost never see them while browsing. When one does see a comment on a page, however, it is usually unintelligible, frequently being either a few words long, in another language, or something like "lol." Also, if one does leave a sticky on a public page and someone replies to it, unless the sticky is on an extremely high traffic page for Diigo users it is usually months before anyone responds to your thoughts, and by then you've forgotten that you posted something there at all (Diigo's ability to inform you of these responses is completely lacking).
* No Colors: Through Diigo there is no means by which one can have multiple highlighter styles (or even choose one's own!). There is one primary one available, standard yellow highlighting, and two others that manipulate the text. If you switch from one to another, all your bookmarks will reflect the change. This feature was, is, and always will be a deal breaker for me on highlighting extensions. I kept waiting for this functionality to come from Diigo, seeing it in some other extensions, but Diigo has yet to even mention this feature for something on the horizon. Highlighting functionality brings me to my next major gripe.
* "Welcome to Diigo": As other reviewers have criticized as well, Diigo usage requires you to be signed into their website, but the website signs you out after any fairly short amount of inactivity. (Oh yeah, did I mention it kinda sucks to have to have an account for an annotation extension? More on that later...) Everything from going through every account setting available for Diigo users to using Cookie Culler has left me where I started on this one. I nearly modded the extension myself, but bigger issues came up with using this extension that left me wanting to steer clear of it altogether.
* Terrible performance on plaintext files: As a hobbyist programmer, I thought it would be fun to use this when examining js, css, html, c++, etc files online to take notes and help me learn more. Unfortunately, I quickly realized that Diigo is of no assistance in with these areas. On large plaintext files that load instantaneously without Diigo, the more one highlights and stickies the text the slower Diigo gets. The performance is awful, though, even after one or two highlights. With more annotations on such a file, one even begins to get Firefox alerts asking if you would like to stop Diigo's script from running on the current page as it is clearly taking so long. All the while, by the way, as with every time Diigo is loading your highlights, the remainder of the browser effectively locks up when Diigo is inserting highlights into a revisited page.
* The BIG One: Diigo saves all highlights locally in PREFS.JS. This might not come as a shock to you immediately. Indeed, I found it reasonable for quite awhile. But consider the fact that Diigo adds each highlight (or at least each URL...I can't recall off the top of my head) into prefs.js as an entry, with the entire highlighted string as the key value. As with another famous preference registry that comes to mind, after adding enough to prefs.js, Firefox experiences a nontrivial system slowdown. I didn't mind having to dig through over 5,000 entries in prefs.js, but I did mind having that many when Firefox began getting far too top heavy. Initially, I was unaware of what was causing the slowdown, but after several wasted hours of extension configuration changes, uninstalls, preference edits, etc., it was clear that Diigo was the source of my issue. This was most abundantly clear after installing a fresh Firefox build, installing Diigo, and only importing my users.js file. The new profile was predictably faster with 40 or more extensions not installed, but it still remained significantly slower than a plain old fresh install of Firefox with the same Diigo extension and preferences other than the highlight preferences.
It's worth noting that simple solutions can be thought of quite quickly for this last problem but none have been implemented by Diigo. For starters, simply storing the last date each highlighted webpage was accessed within prefs.js would allow for tweakers to remove the highlights from the list that haven't been used in ages. (Note: Creation dates are already stored at diigo.com for each highlight, and, as such, this wouldn't be much work for Diigo to implement.) A more user friendly alternative would be to include settings through which one could tell Diigo to remove highlights from local storage (but not from diigo.com) that had not been accessed in X numbers of days/weeks/months. By doing this, only the highlights most needed by the user at present would be saved within his or her browser's preferences, and the remainder could be called from diigo.com (and thus stored again locally) the next time the pages they are on are visited.
These along with a number of other less significant issues have led me to switch to solely using Zotero for research and Autocopy for my copy and pasting needs. I hope Diigo actually starts correcting its long standing and frequently mentioned bugs in the future, but until then I will stick with the other available options.