I used to sell for Comcast, then DirectTV, and then Dish Network. Basically for the money, Dish Network can't be beat, followed by Direct and then Comcast. However as stated earlier, tell them you are going to cancel and they will throw all sorts of offers your way. Just don take the first offer. The usually let you bid your services when they get desperate. Remember Comcast needs to maintain about $78 per customer, so you canusually get close to that - if you play the game right.
Also, if you don't mind playing their games - switch between all three of them taking advantage of their switching offers every two years or so and you can really get great deals.
Keep in mind to ask for every discount you can get or they won't offer them. Act hesitant and make them give you a reason to use their service. The only loyalty you need is to your bank account, because all of them will screw you the first chance they get.