Years ago, their desktop client was the only one among popular similar services that supported proper delta sync while also covering all major platforms. Absolutely indispensable if you used it with things like TrueCrypt containers, where changing one byte within would cause most sync clients to resend the whole 50 GB, or however large it was. Dropbox handled this fine and would finish in a couple of seconds (actually sending new data, not just pretending it did that).

I don't know how it is these days, wouldn't be surprised if other commercial services still haven't figured it out.