I have been somewhat impressed by Mojeek, but it does have two obvious flaws:

1) It not really good for localized search, it might be if you're local to the US or UK.

2) No !bangs. Coming from Ecosia I frequently just do !w !maps !yt because I know where I want the answer to come from

For English language searches, it completely usable, but not quite as good as Bing or Google. I really wanted to try to use Mojeek as my default for an extended period of time, but the lack of good local search makes it a bit annoying.