Can be habit-forming. I wrote an article once on creating virtual dioramas using SU.
Yes, quite addictive at times. But also a great way to relax sometimes if you don't let the Muse take over and force you into doing more with your project than you had planned.
I look now on that arch roof shelter and I can see where I could make some changes to the model to make it better but I think I will leave it alone for now. Trying to not model the complete subject in one sitting can be a real challenge. Worse yet is trying to model the external features without getting bogged down on the internal (never seen) details that you know are there to support the physical shape of the object you are trying to model.
I still claim it is a good way to relax :)