I got a Hisense 12K BTU from Lowe's last summer while in the Seattle area. First time in forever that I'd lived anywhere without central heat and air, and it was a pretty warm summer for Seattle. I only used it in the living/dining area and then had a few fans pushing some air to the bedrooms, etc. It didn't seem to work all that well so I started trying to figure out what was up. It was pretty warm behind the unit, and I found that exhaust air was coming out from between the unit and the plate that slides in and has the hose attached. I threw some duct tape on it and also taped up all the window to spacer joints and together those really helped the machine cool the area.
This year we are back over in Eastern Washington and have great working central AC, so I just had the portable unit stored in the garage. When our bookkeeper asked about getting an air conditioner for her office, I remembered the little portable and took it up. Had to drill a hole for a 4" exhaust (she doesn't have a window!) but after that she's able to get her and my foreman's offices (each about 12x15, connected by a doorway) to below 70. We've been pretty warm recently, so I'm pretty satisfied with that so far!