For me, a local FTP server doesn't work as I'm in a rural area with more frequent power outages than what my clients would tolerate ;-> When I was still in the city, it was a great solution for all of the reasons Chuck mentioned. Customers only ever saw their folder and the whole thing was behind a firewall. Download time to DAW was essentially 'zero'
However, that's not my reality anymore, and I decided that I'd suffered long enough with Hightail and then tried all of the alternatives I could find, finally settling on 'filemail.com'.
In a mastering context, I'm often sending files to the same set of people for approval and the ability to create 'recipient groups' is really helpful in saving my aging wrists from RSI. This simple feature has never existed in YouLostit / WhyTail and doesn't seem to exist in most of the well-known file delivery apps. Filemail's tech support has been great (after Hightail, I guess any tech support looks good).
So, although it may sound like it, I'm not on their payroll, it's just that they seem to be the most sensible and useful of the lot, to me. Price is about the same as a Yousendit Pro account.