Designing Kitchens and Bathrooms - Downdrafts in islands

Are downdrafts really necessary for an island cooktop? They seem really expensive for the number of times I have used the vent over the range I have now. But I am doing a new kitchen and I want to do it right and I suspect adding one later into my granite will not be easy.
thanks
eeprof

Most downdrafts are an integral part of the cooktop, and if you have the ability to put one in, I would do it, as it would help in reselling value. You have to make sure the ducting exits the house properly. And as you said, installing one later may prove to be impossible.

Most drop in cooktop manufatures also sell separate down draft vents. I have a GE downdraft that I installed when we redid our kitchen. IIRC it cost about $650. It is not as efficient as a range hood, but it is still pretty good at exhausting cooking smoke and odors. Especially from the back burners.
If you are having solid countertops installed (granite, quartz etc.) you need to let your installer know up front. I suspect that cutting additional clearance once the counter is installed might be pricey.
If you decide to go with a downdraft you might consider remote mounting of the blower. I mounted mine in the basement which makes the vent pretty quiet. All we hear in the kitchen is flow - no motor/blower noise. A plus is the saved cabinet space.
I vented mine to the outside with 6" flex ducting. Anything smaller will impact on flow. I vented into my unfinished basement for several months until the weather improved without consequence. Unlike range hoods, the exhaust temperature is a lot lower and there seems to be very little grease in the duct (actually none) after 6 months of use.