For me, the purpose of these sites is not the "surf by proxy" filtering that most people seem to use them for. I like the daily scour for information that I currently pursue. These sites give me ancillary links and perspective from the community that surfing alone could never get me. Considering how smart this community seems to be in most cases, it would be foolish of me not to ingest the community's perspectives and additional datapoints that folks provide.
From Kevin Kelly's Cool Tools, Consensus Web Filters. In addition to an excellent write up on the emergence of this category of Web site, it includes a pretty comprehensive list of sites that fit into this category. One great filter, which I have sort of become addicted to, but which is not mentioned in his piece, is TailRank. TailRank has gone from being a site I was not familiar with to one of my ten most visited sites in less than a month.