In my mind Dropbox became a company not worth supporting when Rice joined Dropbox's board (http://www.drop-dropbox.com/). Personally, with a board member who advocates warrentless surveillance it seems unlikely that we share similar views on the security of my data, and I wont be using their service.
So even outside of this surveillance stuff I don't get the point in using them.
Oh, and they always supported the big three OSes.
These last two features makes them stand out against the myriad of alternatives. (Especially the offerings from Apple, Google and Microsoft are laughably bad.)
Dropbox is more expensive but not that more expensive given that it just works.
That said, I switched a couple of months ago to Seafile (the German branch) and it has worked almost as good as Dropbox.
The reasons for switching were: not based in the US, supports more OSes, Rice, cheaper.
Some features like selective syncing do not work as well but others like multiple libraries are a solid addition.
I have run into a Git repo issue on Seafile that I never had on Dropbox and the client on Windows could not sync some files due to the filenames (luckily those could be renamed).