Excellent replacement for del.icio.us Rated 5 out of 5 stars
Even if you don't want all the other great features, it works great just as a replacement for del.icio.us. It's faster, more responsive, and it even sync's with del.icio.us if you want to use both. They also have a Firefox 3b5 version that I've had no problems with at all.
The only downside is that it takes 2 clicks to add a new site. Once to add it to bookmarks, then again to add the tags. I'd prefer a single click since I tend to always add tags.
I'm still experimenting with all the features, but so far I've very impressed with this service.