I have the same thing. It's great. It's the same exact layered material that insulates the Mars Rovers, many satellites, portions of the Space Station and other things NASA.
I also ordered an uncut sheet of it and cut a piece to insulate the roof slope above the headliner to eliminate condensation up there. Where the passenger seat used to be I have a wooden desk thingy, and I have a shelf that opens to the rear where I have a USB hub, store external hard drives, Eneloops, 18650 and CR123 batteries, a few other things. I cut a piece of the insulation to cover the back of desk because the Espar blows right into it otherwise.
If it was remotely affordable, and it's not, I'd have bought enough sheets to insulate the entire van instead of using Reflectix.
A tip... Moisture helps the suction cups stick. Wiping them with a moist towelette is probably best to do that. Second best is to lick your fingers and then wipe the cups with spittle. Third best is to just lick the cups outright. I've never done that last one. The second one is my go-to. But since the area of the window where the suction cups sticks, and the cups themselves need to be very clean and free of any gunky buildup, I do the first option fairly often.
On a hot, sunny day, if you park the van so the window is facing into the afternoon sun, the van will stay appreciably cooler than if the sun bakes the side or rear of the van all afternoon.