Unite is really awesome and I'm sorry to see it go, I used it for quick sharing with friends. Much more convenient than Dropbox when you just want something right there and then.
I think Unite suffered from not being open enough. You required an Opera account to use it, and there's no open specification for building compatible tools and services. What you could build into the browser itself was quite limited. (Plus you had to leave the browser open all the time, which can be a pain.)