Nobody seems to care that you have to be logged in to Blekko in order to benefit from all the personalization features. I would think that would be a dealbreaker for a lot of people.
I love the personal spam filter. I can't see the rest of the slashtags being that useful for most of the searching I do, but I can see myself getting used to it. I go to wolframalpha regularly for things I used to use google calculator for, so at least in my life there's definitely room for niche search engines.