The excellent Roomba j7 is down to $400 this Prime Day. | Photo by Jennifer Pattison Tuohy / The Verge Robot vacuums are some of the most useful smart home gadgets you can buy. Small, circular home helpers that clean areas you can’t reach and work hard so you don’t have to, these little suckers are life-changing. As The Verge’s resident robot vacuum expert, I’ve tested dozens of different models over the last five years and have always highly recommended only buying robot vacuums when they’re on sale. This is because: (a) they’re expensive, and (b) they're almost always on sale — if not today, then likely tomorrow. And today, the last day of the October Prime Day event, is a very good day to pick one up. There are some excellent deals to be had — over 50 percent off in some cases. Here are my picks based on price and prowess: iRobot’s R... Continue reading…