I got Dropbox mainly for collaboration (sharing datasets and R files), and now I use it as the central storage location for all my photos - they go straight from the card (which is then cleared to make room) to Dropbox through it's automatic transfer function. I have had no problems, although the occasional horror story of individual files being lost without a trace has prompted me to start uploading them to a photoblog.
I use Chrome sync[h] but because the computers I use are generally somewhat slow (especially with the number of tabs I am in the habit of opening) I don't often use the extensions that are synchronized. I am not impressed with the bookmark sync[h], as old folders that have been deleted on one computer are often restored from another. Then again, I have somewhat given up hope on keeping track of things I want to investigate with bookmarks anyway, as I create just too many. To-do lists have supplanted them for the most part; I still use Chrome's "save this window as a folder-full of bookmarks" function to save a browsing/work session for a time when my computer is less bogged down.
For the most part, though, I have been trying to eliminate the need for backups altogether. As a student with not much budget for purchasing memory, and one that uses temporarily loaned computers and ones that break after only a year or two of use, I find it much easier to use online services for most program and data storage - using Google Docs rather than Word or Open Office, for instance. It makes collaboration and sharing a lot easier, too - I can worry less about file formats. To pick another example, instead of using iTunes or WinAmp or VLC (although I also have the latter for miscellaneous purposes) with a music library I use Grooveshark.
There are still many things that need to be offline due to the paucity of Internet access in my house and sometimes at school, but many things can just be re-found - it is easier for me to re-download my ebooks, and various programs (Pidgin, GIMP, Inkscape, Notepad++, Chrome of course, a tuner program, and others including those mentioned above [Dropbox and VLC]) than to find and transfer them on a jumpdrive or such. However, I noticed I have also taken increasingly to putting all my files in one place - a folder on the desktop - rather than using My Documents. I even run programs that do not need to alter the registry and therefore self-install, such as tkMOO, from the desktop. With all this centrally located it is easier to pick up and move shop should I need to.
And now I have a website I can put stuff I don't mind being public in one place, too.
This all might be oblique to your question since I am not using the pay Dropbox, or Dropbox in a big way at all.
-Arlo James Barnes