Ive completed the campaign 2x once on my first of course, which was a barb, got him to like 54 or some shit and found out how weak the bleed build was I was using in T3. Need to respec his ass. Then rolled up a druid and forgot to click the skip campaign button so ran the whole campaign again, got him to like lv60 and got bored. Honestly didnt mind the campaign while leveling. Then rolled a necro with no campaign and still playing him and hes like 59 right now. Still going strong.
Even if you forget to hit skip campaign when you make a new character.. There is still an option after the fact at the login screen.