* It 100% "just works" on all my devices (Mac, Linux, on a server, web, on my phone -- hell, I could stick it on a Raspberry Pi!).
* It integrates seamlessly with my filesystem.
* It has never, not once, failed me or bugged out on me. I've been using this product since it first came out and have not experienced even a minor bug!
* It's not tied to any other platform or service that might have conflicting interests.
* No fancy "smart" features or unexpected behaviour. All options are explicit and behave as expected.
* No fancy UX experiments, just straightforward to setup and use.
* I never have to think about it, it just does its job.
* It's a paid service that makes money (this in itself IS a feature).
It's just better than the competition. The peace of mind and get-out-of-the-wayness is well worth the $10 bucks a month. One of the few products that would be truly painful for me to lose...