Product review (1 stars): Weak
We had some issues with the installer trying to re-use the 6" ductwork from the electric downdraft we had before.. but even after getting them to come back and upgrade to 8", the suction is still pretty wimpy on this.  Which is nuts considering it costs as much as the cooktop itself (once you add the blower). (we got the 600CFM inline blower).

I'm not saying there are good/better alternatives.  Based on what I read we did was well as we could by getting one of the taller models. I'm just saying not to expect much.

update1: the design flaw (if you want to call it that) the "better" version of the cooktop has the powerful burners in the front... if they were in the rear (or at least the middle, like the lower-end version of the 36" cooktop), the vent would have a better chance. Essentially the vent is good enough to "bend" the fumes/smoke from the front burners "towards" the unit, but it just bends that direction and still goes over the top.  It's not effective enough to capture.

update2: as we head in to winter, I'm noticing another big drawback... this is a big funnel for cold coming back in to the house.  Installed in the island, running ~15' of ductwork to vent on the foundation... the cold air comes in the duct (and/or the duct is sitting in a cold crawlspace) and while the air itself doesn't "leak" it chills the entire metal assembly (inside our kitchen island), which is then 5-10 degrees colder inside the island than the rest of the kitchen.  If it were more effective as a vent, I'd be less disappointed.