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I'm researching this topic too. General consensus from friends I've spoken with is that the integrated hob/downdraft is good, although noisy. I'm now trying to figure out if I can fit a built under oven below one.
The other option is to have a retractable downdraft hood behind the hob, From what I've read these are slightly more effective than a downdraft hob but additional complexity with install, also more cost and space used.
Any first hand experience of downdraft hobs/extractors here? Are they shit? My suspicion is they'd get rid of a bit of steam but not wok/skillet vaporized spray type stuff.