My personal experience with Comcast is relatively limited, extending to short stretches of time spent at friends' houses. However from interactions with other friends who have used Comcast, they typically praise Comcast for reliable and high-speed service, with AT&T generally bearing the brunt of their less-praising remarks. Comcast has not shown me any reason to view them as an unreliable service-provider.
I have Comcast and personally I have to say I'm happy with the service. It never goes down for me and my speeds are always fast, usually almost over 10 mb/s. Having a high speed internet connection is very important to me and it's a necessity in my everyday life.
A simple Google search reveals a litany of unhappy reviews describing in great detail Comcast's customers' experiences with the company. Accounts of nightmarish customer service, terrible internet performance, and exorbitant monthly rates do little to reassure any potential future customers that they get quality, reliable service from Comcast. Here is some free advice: try another provider.
Depending on what the definition of reliable it is, Comcast may or may not be reliable. If you are okay with regular service outages on a continuing basis then I suppose Comcast could be considered reliable. Otherwise Time to turn off the TV and start reading some books and newspapers.