After getting tired of the slowness and lack of features of Connotea I decided to try out CiteULike. My only regret is that I wish I made the switch about 6 months ago, since CiteULike seems superior to Connotea in almost every way.
In particular, I really like the built in priority list for indicating the level of importance for me to read a certain article. This is much better than my old Connotea method of using a "to_read" tag. Also, the use of tag clouds (instead of simple lists) not only for my personal tags but also for authors is a really nice feature. I was really surprised to see that you can upload a pdf of the paper being cited for storage on CiteULike. I'm not sure how much I will use this feature, but it is a nice option for those papers that you might not have easy access to all the time. Lastly, the search function is simple yet effective and the site runs smoothly without any waiting or timeouts.
The transfer of my bookmarks from Connotea to CiteULike was easy. I simply exported my bookmarks from Connotea in RIS format (which of course took about a minute of waiting) and then imported them into CiteULike. The only small hiccup is that CiteULike doesn't handle multiple words as a single tag (ie. no spaces allowed), but CiteULike did convert all of these cases using dashes instead of spaces.
Overall, I am a happy social bookmarker again and you can follow what I am reading here: