[1] Actually works, reliably
[2] Works on Linux
[3] Generous free quota
Of course, I'm probably not the best customer profile to target. I'm a shameless freeloader, and would never even consider paying money to go beyond a free quota. Until my home Internet service is an order of magnitude faster, these services are only useful to me for simple file sharing and photo backup.
Truth be told, I would drop Copy.com and consolidate around Google Drive if they had a simple "automatically backup my cell phone pictures" feature. I can get Drive to work on Linux, and that's already where most of my word processing documents reside. Unfortunately, Google's photo backup feature is tied to G+ rather than Drive, which makes it useless at best.