It is often a crap shoot. General 800 support takes a long time and it's hard to accomplish anything. I only use the call-ins for local outage to get a truck roll. General services (phone upgrade, plan change, set top box return) I always go to the store. They have usually impressed me. Online appointment scheduling means I never have to wait. They change my plan and send me a confirmation email summary while I am standing there. Really good service experiences for me.
But for weird, infrequent or almost broken stuff the reddit/r/Comcast_Xfinity group is the best. You can pose a question to them: Where is my data usage web page? Why does my modem drop out on Thursday mornings, but back by 2:00PM? When will my area upgrade to mid-split. And they will get back to you sometime later. You don't have to wait on the phone and hope you get a good CSR.
For me, the website feels like the most cumbersome, most risky place I could go to make a service change. It won't show me all the plans available (since I am an existing customer). Pages load slow. Everything feels like an ad and an up-sell. I feel like whatever change I clicked on they are going to secretly add more TV back to my monthly bill because I missed a checkbox.