Become a MacRumors Supporter for $50/year with no ads, ability to filter front page stories, and private forums.
I never get the "Do you want to send this attachment with MailDrop" dialog box, but it's working.

I ran a test with a 21.5MB attachment.

With MailDrop enabled, the message takes just a couple of seconds to send, and you get the swoosh sound almost immediately. With it disabled, the message takes about a minute to send.

Also, there is almost no activity in the Activity window when you use MailDrop, but the old way you see a "sending attachment" item in the Activity window, and then a "Caching attachments" item in the Activity window.

Whatever is going on behind the scenes is good, but I'm not a fan of Apple hiding the interface completely from the user.
 
Those updates are really neat. However 5GB is still not that much. If you need to move more, I'd rather recommend Filemail. They have a very simple & clean web app that lets you send 30 GB to any e-mail for free and with no annoying ad-infested download pages, unlike some other free upload services. Hope this helps someone ;)
 
What's happening... ?

This thing really needs a progress bar... very disconcerting to have clicked MailDrop for a 1.25GB file and then: nothing....
 
Register on MacRumors! This sidebar will go away, and you'll see fewer ads.