On the second day of our rental weekend, the weather was much less perfect in the sense of gosh it's nice let's go outside, but it was absolutely perfect for one wanting to try out the bike in "typical" Portland weather. That is to say, it was gray, a bit rainy and a bit windy. My mom, still staying with us, was up early and wanted to go for a coffee and to pick up some breakfast pastry type snacks. I convinced her that it was imperative we take the Bak, and so, with some trepidation, she climbed into the box and settled in under the raincover. The trip was just about two miles and after some initial worries about instability, she managed to relax. From the rider's perspective, the rain shield did a great job keeping my legs dry and blocking a lot of the wind, I hadn't ridden enough to tell if it made the bike feel heavier or less balanced.
After getting coffee the rain and wind had picked up quite a bit - my mom had absolutely no trepidation anymore about hopping into her "snug little cocoon" to enjoy her coffee and a nice dry ride towards home and Annie's Donuts; I think she may have muttered something about it being a shame I had to ride out in the rain, but I don't think she was offering to give up her passengerly duties.
It rained off and on the rest of the weekend, which was fine by us as we'd gotten at least one day of very nice, dry biking weather at the beginning. We took turns taking each other around the neighborhood in the bike and on a few other short errands. On our last rental day my wife rode the Bak back into inner SE Portland where we met for lunch (I had to work) before she returned it to Clever Cycles. It was rainier and windier and she seemed pretty tired, but claimed to have a had a great time on the trip.
By the end of the weekend we were pretty much hooked on the idea of a cargo bike in our near future, and we were leaning very strongly towards a long-box Bakfiets or something very similar.