I don't want to sound like the "great defender" of DirecTV On Demand, as I don't own stock in the company.
I read these posts/threads and have had the same problem.
Once I changed my ISP, 5-6 years ago, I haven't had one problem with On Demand.
With 6 Mb/s service HD wasn't always 1:1 and mostly 1:1.5 [for every min of program it took a min & a half].
With 12 Mb/s service HD is mostly "watch now", though I've had a few buffer messages that I clear and continue without problems.
I did find one HD program recently that I couldn't watch now, so I downloaded it and it took twice as long.
Like most, I thought it was a DirecTV problem. I checked out the bit-rate of the program and found it to be 14-16 Mb/s, so there wasn't any problem on the DirecTV side as it was coming through at 7 Mb/s.
Not having a connection faster than 11ish Mb/s, I can't tell if DirecTV ever goes any higher.
It just seems 99% of the time it isn't DirecTV that's the problem.
Sure anyone can have a server problem, so that's the 1% and maybe if the program is "hot" everyone is trying to watch it at the same time.