I don't think I am a typical user, but I use Dropbox because they put the same amount of polish into their client for all systems I use (Linux, Windows, Mac, and Android).

I fear that other providers will neglect the client on platforms they are not trying to push. If a new platform were to pop up and become popular, I am pretty confident Dropbox would make a client for it, whereas the others I think would drag their feet. I don't want to be weighed down by my cloud sync provider like that.