So far, I have sent out over 300 orders for DD2 using Sendspace and/or Dropbox. I have paid accounts with both these services, so I am not aware of the problems that may happen to those who don't have accounts. 
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Details
SENDSPACE--When I was away from my home computer and a video order came in, I had to use Sendspace to send out the download links.
Several people who downloaded them complained that the video seemed to freeze at some point during playback. These orders were sent out again, using Dropbox. 

I was also told that downloading DD2 from Sendspace took a long time to download. I'm familiar with that--I've had to wait hours for things to download, and it's no fun.

Two people said that when they tried to download DD2 from Sendspace, they were prompted to pay for more space. What's up with that? I thought I was the one who was paying.

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DROPBOX--I used Dropbox to send video links, as much as possible. It went relatively smoothly, and seems to work.

One person who received the Dropbox invitation wrote to me, and said that he preferred Sendspace instead. So I sent him a Sendspace link, no questions asked.

Another person had trouble locating the Dropboxes I had sent him. Although they appear in my own box, and say they are accessible to him, he couldn't find them. When I made another new Dropbox for him and sent it with the 3 videos he had ordered, this time he was able to find it. 

But then, he wrote back, saying "When I open the Dropbox, it prompts me to buy more space. OK, I get it. I buy vids and I have to pay Dropbox or Sendspace more money. I hate this." 

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Well, speaking as Animan, I hate it, too! As a customer, I've been tricked out of my money on a few occasions, and I vowed that no Animan customer would ever have to go through that living hell. And yet, here's someone who paid me money, and seems to be getting hassled by these downloading "services".  

Since I pay for these downloading spaces, I don't receive any sort of prompts or messages. But they apparently appear to the regular Joes who try and use them. If anyone reading this has anything constructive to add, or wants to tell about their own experience, please feel free to send in a comment. Thank you very much!