I also took this course at UW, about a year ago. It was as you described - online, open-book exam in set location or with proctor, etc.
Despite being quite terrified of taking a stats course, much less online, it ended up being my favorite course and my highest mark. Probably a big part of my MSW admission. I recommend it, but suggest only taking an online stats course if you'll have the time and persistence to sort everything out each week. Stats can be rough if you fall behind in a concept, since a lot is cumulative. Don't be afraid to call the prof, scan & email any work you have trouble with, etc. Also, whenever you get a question wrong, try to verbalize the steps you took and figure out exactly where you went wrong - talk this through with the prof too, if you don't see where the problem was. (Sometimes, you can even get bonus marks this way!)
Anyways, I'd be happy to share some tips and more thorough thoughts via PM, if anyone's thinking of taking the UW stats course. I totally ended up nerding out over it, so feel free to reach out - I really don't mind!