I've been using YouSendIt, now called Hightail, for a few years and have been very happy. It's quick, clients get the email link and don't have to navigate the ftp stuff etc.
Dropbox has been mostly good for me, but it does get confusing with having to invite people to share the files, and I don't like the lack of feedback as to how long a particular transfer is taking.
For instance I can drag a 200Mb file into the folder on my desktop, and it shows up right away. But I know it takes longer than that to transfer so I gotta guess.
HTH