Not as little as you'd think, not as widely as it once was. But the reasons for that have little to do with the "but why?"s thrown at it when it was first promoted.

The “why” is effectively that they got Sherlocked by every platform owner out there - Dropbox’s declining usage isn’t an indication they were wrong.

I still use it (and pay for it) because it is the only option that offers a Linux client.