My current place of employment has Comcast. We have a major outage every couple months during which all work ceases. I rely on an Internet connection for everything I do professionally so I stay away from cable because stability > speed.
Meanwhile I've had Comcast residentually for about a year now without any noticed outages. At work we've got a local ISP (who we actually do design work for!) and our internet goes down once or twice a week at least.
There's really no point in saying a nationwide company is reliable or not because it's incredibly region-specific. At home I've never had connection issues, I generally get the speeds I pay for, and I've never had any billing issues. But you ask someone who lives 100 miles in any direction with Comcast and they'll have a completely different story.
Funny story: my employer switched to Comcast business because our old DSL line went down about once a month. We've had one outage in a year, but everyone got hit by that.
