Usability-wise, it seems fine. I'm not sure how useful it is, though. What's the value of sifting through six different sets of search results, as opposed to just using Google? If I don't see what I'm looking for on the first page of Google results, I'm more likely to click through to the second or third (or sixth) page than I am to switch to a different search engine, even if the switch is as simple as changing tabs.
Well, it depends what you are searching for it may not be the right tool for every job, but neither is google. There are certainly situations where multiple engines are useful. Try out the craigslist group for an example, or the Library group.
