Social Bookmarkers - which one do you use?

With the use of FireFox Portable comes the issue of syncing bookmarks. While there probably are apps and FireFox extensions that can sync your bookmarks, all of these (plus the bookmarks) take up more and more space on the flash drive. So I've decided to give "social bookmarking" a go. The thing is, all social bookmarking sites are not alike. Digg.com, while I do use it, is more of a popularity contest than a bookmarking site. Del.ico.us is a good option, but the colors and layout are harsh on the eyes. Since I've used my browser for bookmarks up until this point, I want my social bookmarker to have a few basic qualities:

1. Hierarchy. I want to be able to organize my bookmarks not only by tags, but also by folders and subfolders.
2. Easy on the eyes. Using a custom skin would be nice, but I'll settle for something that doesn't melt my eyeballs.
3. Privacy when I want it. There are bookmarks that I want to keep to myself. Mainly the syllabus pages for my college courses.
4. Buttons! If at all possible, I want an in-browser button that I can click to bookmark the current page.
5. Import/Export capabilities. I don't want to have to add my bookmarks manually. And, should I decide to change services, I don't want to transfer them manually.

Yes, I know, I'm picky. But you already knew that :P When I first thought "Hmm, I need a social bookmarking account," naturally the first place I looked was Del.ico.us. Importing the bookmarks was wonderfully quick and painless, I got some neat FireFox buttons during the setup process, and privacy comes standard. However, as I mentioned before, the color choice was painful. Light blue and gray on a white background? Ow! Worse yet, there was no hierarchy option. The next place I went was to digg to try to find a list of social bookmarking sites. From there I found a rather extensive list of bookmarking sites. Skimming through, the first name that caught my eye was Ma.gnolia.com (or maybe it was the pic). It had all of the Delicious pluses (though I had to search google for the buttons), but it had a nice color scheme and layout. Downsides: it took longer to import my 'marks and, still, no hierarchy feature. Skimming down the same list, the image for Spurl caught my eye. While I'm still waiting for my FireFox bookmarks to be imported (I'm supposed to allow 24 hours on this one), I've added this blog as a bookmark, and there appears to be a decent hierarchical feature. The colors and layout are nice (I like blue-black, and the layout is moderately spacious). A Spurl FireFox addon is available, and seems to work well. Privacy is an option, though the option isn't as obvious as I would like it to be after I've submitted a 'mark. Overall, (and if my 'marks ever get imported,) I think I'll be sticking with Spurl for now. Though, of course, I'll still use Delicious to boost my traffic ;) On a usability/design note: At the time of writing, Spurl's front page says "We are currently forced to offer reduced functionality due to heavy spam attacks." Actually, maybe they should keep this "clean" front page. I can imagine what the front page would normally look like, cluttered with 'mark feeds, but I like the near-emptiness that is currently "up". If I want to see the latest, hottest bookmarks, I'll try to find a page that offers them. So, what bookmark service do you use? If you're not satisfied with what you've found, what do you look for in social bookmarks? Also...anyone know a good RSS aggregator that respects my preferences?