When I saw Google released this Personal Blocklist extension, I tried it out in Chromium immediately. I would love to help improve the search result, punish those crappy websites.
There are many garbage-class websites around, they really need to be spanked. You bad boys! Some just like what this extension intends to target, the content farm. They are just awful, they fake content or gather other's content. Those website owners are shameless.
There are another type of websites, archive type. A typical case is mailing list archive. I don't like this type websites, either. Often, they outranks the originals in search results.
Another type is software download website. I have very low score for software download website. Partially because I don't need those websites because I am using Linux. I use either package manager or compiling by my own. Some of those websites scrapes FOSS hosting websites such as Google Code or Sourceforge, even the Chrome Store (formerly Chrome Extension).
Some websites I classify as rip-off. StackOverflow rip-off, Google Groups rip-off, usenet rip-off, manpages rip-off. I dislike those, they have nothing original.
I am hoping someday, I will see a real improvement in search results.
As of the extension, I have been wanting Google will release same for other browser. But they haven't and I don't know if Google will. Chrome is only third popular and no web browser dominates the market. Though Internet Explorer has 40%+ share, according to Wikipedia, it does still not count as domination. I believe the number isn't the true value reflecting everyone's preferable browser.
The blocklist is stored and processed on your computer. If you recall, there is some period time that Google allows you to delete certain page from its result. I think it's a search experiment. It had a cross icon next to page title, you click it, then you will see a Puff animation. Along with that cross icon, it's a Up arrow, which allows you to pin a page, just like the current star icon.
But it's removed for quite some time. If I recall correctly, the process is on Google's server. It's per page basis, not like this extension, it's domain-basis. I really like the feature, too bad it's removed.