"Pop-up" or downdraft venting is chosen when there is no other option due to existing construction. It is not nearly as effective as a hood. You are working against the laws of physics with a downdraft, because grease and smoke drift up (where a hood would capture it), but a downdraft pulls down. Furthermore, if you have gas burners, you will be pulling at those flames as well. If you have the the ability to install an overhead hood, you should choose the hood instead of a downdraft without question. Why are you looking at a pop-up/downdraft?

I don't have any specific data, but I have read in a number of places that downdraft vents are ineffective. Consumer Reports does not even review them, for instance.

I don't want to go the pop-up/downdraft route for the reasons you state. One of the plans we might have to go with would be to do a 'mini' remodel and just go with changing out our electric cooktop and putting in the Blue Star range I want. Our concern is where we would center a hood for future finalization of the remodel. (It's hard to explain the complete plans to the complexity). My husband came up with the idea of the pop-up/downdraft vent. We currently have a Jenn-air electric ceramic cooktop with the downdraft vent and I HATE IT. We just wanted to see what others out there had to say...

CR only tested 2 models, one rises 8 1/4", the other rises 9 1/2". This is just marginally more than the 7" rise height of my current 16 year old pop-up. There are a few pop-up models that rise 10" and 14" and supposedly a new one from Bosch rises 17". They sound good in theory and anecdotal feedback from fellow GWers bears that up. Unfortunately, until CR tests the taller rising units, it's hard to know for sure if that the extra height sufficiently offsets the downsides CR lists and merits the additional cost.

That said, if we had stuck with a plan that called for a cooktop in our island, a tall rising downdraft was definitely high on the list to meet venting needs.

the two pop-ups were rated fair for smoke, very good for volume and good for noise.

I have the Thermador 15" downdraft, and I love it. It easily clears the top of all of my pots, and I like that my room space isn't cluttered by a hood over the island. In the four years since we installed it, we have no residue on the ceiling or cabinets, so I think it's doing a great job (plus I can see the fumes getting pulled in). The only limiting factors I see are (1) you have to cook things you want to vent on the back burners-- not an issue for me because that's where my most powerful burners are-- and (2) I had to allocate space for the blower in my island. Hope that helps!

I have a KA downdraft with a 14" rise behind my island cooktop. Works well for what I do (no grilling and only occasional searing.) I have a skylight overhead so a hood was not an option. I'm not crazy about having anything close above my head, either.

Before the KA downdraft, I had the JennAir cooktop with the built-in downdraft. Now, that was useless! The higher popups are much, much better.

"just go with changing out our electric cooktop and putting in the Blue Star range I want."

This is actually forbidden by just about every downdraft mfg. The two that permit installing a downdraft with a range only support doing it with a range they also manufacture.

Sometimes its because they physically will not fit together - a cooktop is normally fairly shallow and the downdraft hood has a top of sheet metal underneath it, some duct work and sometimes a blower - a range just won't fit into the same location with the same hollow places underneath. Sometimes its because of the uninsulated electronics and how-where the range oven vents its excess heat.