I used to think the same. I have since used a down draft with a gas cooktop, when staying in in a vacation rental. Given my low expectations, I was surprised how well it worked. It still wouldn't be my first preference though. It very much is a compromise, and you can tell. Not a great idea on combination with an open floorplan

You won’t have many choices for a cooktop with integrated downdraft. You’re probably looking at this KitchenAid or the Jenn-air version as a choice:

http://www.kitchenaid.com/shop/-[KCGD506GSS]-6004597/KCGD506GSS/

Anything more than that you’re probably looking at a larger kitchen remodel and it’s usually a snowball effect of replacing a lot of cabinets and appliances. So the above cooktop would give you gas and a newer downdraft.

I've got a GE gas downdraft cooktop. I wouldn't recommend. What a royal pain, the fan draws the gas flames toward the downdraft. Furhter, it took a lot of tweaking to get the thing to work properly. The GE morons set the minimum flame height by default at nearly the full output (which admittedly isn't much on this unit).

From what I understand, you want a downdraft that raises up when in use. And the higher it raises the better. That's still not ideal with gas burners, but it solves some of the issues with drawing the flames to the side

And it requires a great deal more cabinet room, and expense. And still isn’t the functional equivalent of a low end overhead hood.

If you have to have a downdraft, you are infinitely better off with a nice induction (not to be confused with smooth top radiant electric.) Induction allows for ease of control like gas or better and the easiest clean up ever, since nothing but the pan gets hot, not the cooking surface. Induction is a cleaner technology, so there is less stuff to suck out of the air and you won't have the flames of the gas hobs being affected by the suction.