Can't speak from personal experience, but I did have an acquaintance who had one. She said she rarely used the vent, as it was pretty much useless with the gas flame. The airflow used to pull the cooking grease, smoke, etc. also draws at the gas flame, creating (at best) uneven, ineffective heating, and (at worse) a hazard. She was in an older, multi-level condo that did not permit alteration for outside venting.

Her solution, as a stop gap til she could get an electric range, was a set of top-of-the-line, stainless steel, fine mesh spatter screens for the pots and pans. She said they did a pretty good job, except they do little for cooking odors. She turned on the downdraft when finished with the burners, which helped a good deal. Bonus: spatter screens are silent.

Completely useless anchor. Get the cooking zone on a different wall where you can use an overhead hood. It will likely be a better layout as well.

I have a gas Jennaire downdraft...exactly what kchreg said! . However, this is a large home with 12ft ceilings and even though I cook a lot for two people, I can't say it's bothered me an awful lot over the last 9 years, and I'm not finding grease spots anywhere. I would much rather have the cooktop in the center island than the sink, which was my only other option.
I find that the broiler in a separate wall oven creates about the same amount of cooking smells as the cooktop. I suppose a lot depends on what kind of cook you are, and what you are cooking.

I got one, get a unit that rises at least 12 inches, they have come a long way, I've had 3 houses with the island cooktop set up, you are going to have to pay 1500$ for just the downdraft unit alone.

So you are going to pull hot air and grease downward - I believe heat rises. You have at least 2 - 90 degree turns in the duct. Downdraft is useless.

I've used several over the years, none that actually did anything useful. None were as tall as the one pictured above, but even so, without the capture volume overhead it seems that you'd have to have a truly heroic flow rate to create horizontal laminar flow to capture stuff that wants to head for the ceiling. I think these things are fighting a losing battle with physics.

Hey rwiegand - I used to work at Langley, with the larger wind tunnels - do you have a license to use the phrase 'horizontal laminar flow'? :)

What , who cares he is right heat rises and IMO does not more well sideways , I have had a downdraft install completely useless IMO.I would much rather have a sink in the island than a cooktop anytime.

Check out the Kitchens forum an the Appliances forum for more info.

Thank you for the feedback. Consumer reports state the same opinion. Generally, they are not effective.

I have the same Thermadore downdraft as ninigret and have also been pleased.

Heat doesn’t rise. Hot air expands and becomes less dense and rises. Heat flows from hot to cold, regardless. (And coldness is not actually anything but, like darkness, the lack of it.)

This sounds pedantic but really matters when people start talking about insulation.

Everyone with TALL telescoping downdrafts appear happy in reviews. Those without don’t like them. Also, people hate Broan downdrafts. Go figure—they like Broan bath fans.

"Hey rwiegand - I used to work at Langley, with the larger wind tunnels - do you have a license to use the phrase 'horizontal laminar flow'? :)"

I'm a biologist, so my experience is with hoods for biological containment. The horizontal flow ones don't work very well for that either. They've been banned for anything dangerous and don't do a very good job of preventing contamination.

In Hong Kong studies of household hoods over woks, the side walls/containment were the most important issue.

Unfortunately, US domestic style hoods just don’t have much research. :(