I've had DSL since...ugh...it's been awhile, maybe 5-6 years now at the house, the FD I ran at had it in both stations for 2 years, and I've had it at 4 locations in my offices over the past 4 years.
At the house we've gone down a handfull of times at most:
Twice when the phone connection to the house went bad, Two were caused by careless drivers deciding that the telephone poles either didn't need to be there or who just wanted to test the safety of their vehicles by driving throught them (FD), and a couple of times when power went out.
The FD ones never went down, then again the telephone company was next door...so that may have been a reason. To my knowledge our offices never died either, of course those were business class and had priority over residential services.
Then again, the two from the line connection at the house were the only ones that cable would've prevented MAYBE...but the others would have killed cable one too. I can tell you the Cable TV service here sucks, goes down all the time...so I would assume the internet would with it.