I like their site design a lot. Definitely still pretty far off from shapeshift, though.
With so many trading pairs, if they are legit, they could definitely be a potential replacement or alternative to shapeshift.io.
My primary concern with them though is the same as what happened to shapeshift, which was having to add an account system due to regulatory pressures with everyone needing to conduct KYC in order to trade. This could happen to faa.st in the future as well. Also, we don't know if this site is actually a regulated entity, so you never know. If issues come up, you don't really have a lot of options.