You will be using one of DreamHost's SMTP servers, but you can use mail.mydomain.com to access your mail.
The DreamHost Admin Panel is home grown and quite a bit different to cPanel, but it offers all the functionality you could want and personally I find it easier to gets things done (compared to cPanel). However, in the past it has suffered from speed problems, which seem to have been resolved now.
Judging from reports here on the forums, downtime can vary considerably depending on which server you are located. Personally, I have seen very few site outages since I've been here and the duration of those was only short (about 20 minutes being the longest).
My experience with hosting streaming video here at dreamhost is fairly limited, but a few of the sites listed in the wiki article below contain streaming video, so you can see the performance yourself.
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