Yes the vog was a hot issue some months back but now it is not new news and I think many are just used to it and go on about their daily business...I myself have tried to check sites like the volcano website once or twice and ran into -- no clear cut information -- that can be understandable because everything changes all the time...
How it will affect an individual depends on the conditions of the day, where you are and your personal sensitivities or health. We have been having a lot of rain during the summer of 2008 which tends to wash the "vog" down out of the sky.
The volcano is way South so areas like Hilo, and especially Puna can have more density. The winds vary and the density varies so it sometimes gets blown to Oahu.
There is no clear cut answer. The Kailua-Kona area certainly can see the vog but the air quality is not anywhere as seriously affected as Puna, which has had evacuation alerts. It will also depend on many factors like elevation (more likely the vog will settle lower) and if there is wind and how much gust there is. When we come into town we notice pockets between Honaunau and Kailua that are more noticeable as we drive thru. In town (Kailua-Kona) we do not notice any vog issues from breathing. Up where we live same thing, the air quality is very good.
A recommendation would be to avoid Puna, and if you think you can detect vog as you drive thru some areas, roll your windows up and air condition for that time period. It might be worth considering asking about filter systems in a car before you rent. I know our Honda CRV does have a filter system when the air is switched to “recirculate”.
Our area where you would be staying in Kailua-Kona at our home is currently not in a bad air quality area and when we are there it seems fine. It can be the connections between areas as you travel that could affect you if at all. People vary so much it is hard to predict.
What effect is there? If you breathe dense vog it can cause you to get flem (mucus) in your throat -- I guess a natural reaction of the body to collect and remove the particles. Some people report they can feel it in their eyes and need to wash it with eye drops. Some people report it gives them a headache and take an aspirin. Solution is to avoid the dense areas, if you do drive thru an area and think you are in some vog, close windows and air condition for that stretch, perhaps take eye drops and have some aspirin with you.
Perhaps consider staying away from low elevations when Vog is more apparent? Certainly avoid far South locations like Puna when the Vog is apparent?
I would suspect if you have COPD or other lung/breathing issues you might be much more sensitive.
What you might want to do is go to the "visiting Kona Forum" at http://konaweb.com/forums/visiting.cgi and read or ask/interact for more opinions. I glanced (July 2008) and there does not seem to be a lot of talk about vog but people are talking about seeing lava flow.
Current and recent guests have not mentioned issues to me about air quality, most are more interested in the high gas prices and their ticket prices.
We have had vog around Kailua-Kona and the Big Island now for quite a few years. The main thing it does is limits the distance you can see the landscape and the sharpness of what you can see.
For us living here, our solution is to air condition thru sections we may need to drive thru, and otherwise it is inconvenient in that we cannot see as much of our ocean and sunsets as we used to. But then I can remember 26 years ago before the volcanoes started erupting that you could see much further into the mountains and see more clouds, etc. So I guess if you compared now to 26 years ago there would be a much more drastic change, more nostalgic "the good old days" stuff than anything else. There is still a lot to do and see and good things like more services, big discount stores like Costco and Wal-Mart, cell phone towers all over we did not have 26 years ago, so it is a tradeoff in that respect.
The volcanoes could calm down to hardly nothing any time, then they could go on for 20 years (like my 26 years ago example). I've heard they are not pumping as much as a month or so ago -- but rarely do I keep up on it when we do watch TV it is our regular shows or movies not the news.