I've been playing around with it here and there for the past fews days and find it quite interesting.
User can create "slashtags" and add sites to them to create their own vertical SE or use some of the prebuilt ones.
The SEO research part is pretty cool though at this time the index isn't as big as it could be so the data isn't complete.
[example.com /seo] returns a nice overview. You can drill down to linkage data that includes anchor text, suss out dupe content, compare data for different URLs and quite a bit more.
[this keyword phrase /rank] breaks out the ranking data for different factors in the returned results. It will also show the ranking data for different combinations of the search: this, keyword, phrase, this keyword, keyword phrase, etc.
You can even use it to find, say, a competitor's network by using /adsense=insertidhere.
What I find really interesting about Blekko is that they are open to user communication - and do not keep their ranking factors secret. I think this is more in keeping with the spirit of the internet. It will also get them a lot of users and links from a frustrated and very powerful and influential SEO community.