I agree with Melbel. If you're a casual user answering questions and doing a few hubs, I guess it wouldn't be so addictive - more like something you always intend to give more focus. If it's something you're passionate about, however, such as installments of a book (like mine are), short stories or articles about your love (such as gardening or food) then I think it becomes incredibly addictive, even more so if your articles become popular.
I think this is also the key element - popularity. People who regularly receive positive feedback for their hubs want to keep doing so, as it is maybe an affirmation of their talents that they don't receive in every day life (what other forum allows you to give out your creations for peer reviews and appreciation?). In this sense it can become so addictive.
I don't think this is a bad thing though. Not only are you progressing forward with an activity you love, you are interacting with real people, sharing knowledge and interests and being part of a community. As long as it doesn't stop you eating like Melbel said then it's a great thing to be involved in I think :) Although, when you start dreaming about it, you definitely know you're addicted ;)
Hope that helps!