Update below: Quintura has just about everything you'd want in an alternative search engine: Solid results, a nice interface that's intuitive and social features to share searches with friends. However, Quintura (see gallery for details on how it works) isn't easy to add as a default search into your browser. Those hurdles may ultimately limit adoption rates.
As I've noted before in this life without Google series, the toolbars in your browser are critical to search engine usage. The days of going to a destination and conducting a search from there are long gone. I'd bet most searches are conducted from within the browser.
In Firefox my usual method of adding a search provider is to go to the MyCroft Project to add one. A MyCroft search on Quintura turned up nothing. So I went directly to Quintura.com and found an "Add to Firefox" link. That worked fine, but you have to go to Quintura first to add it as a search option.
In IE7 adding Quintura (gallery right) was easier, but there's also a catch. Microsoft allows you to add any search provider as long as you do a search on "TEST" and cut and paste the link into a box to install. The problem: Finding that link isn't easy on Quintura since the URL in the browser stays the same. The way to get the TEST search URL is to pretend you're sharing the link with a friend, which generates a URL, and then pasting that address into IE7.
In any case, those hurdles aren't huge, but you do have to be motivated to add Quintura to your browser. If those hurdles are eliminated Quintura could gain more traction because it's definitely a keeper of a search engine.
The visualization capabilities in Quintura primarily rely on tags in a cloud much like other Web 2.0-ish properties. With the tags, it's easier to see search relationships in Quintura. In fact, I had few gripes about the search engine and would definitely use it again. I also like how Quintura is targeting verticals such as a search engine for kids.
As far as user interfaces go, Quintura served up one of the better experiences. Shame I had difficulties adding it to my browsers. When you're trying to post just a smidge of market share from the giants there can't be any hurdles to making a search engine a default choice.