My family switched from Deet to Lemon Ucalyptis last year due to health concerns my wife had (esp for the kids). We gave it a good try (several months) - but didn't have good results.
The lemon ucalyptis worked OK to prevent misquito bites in open, breezy areas (which all seemed to have only small misquito populations) - like the soccer fields, playgrounds, etc. Personally, I only get a 3-4 bites an hour in areas like this, but my wife & kids who use strongly scented shampoos and body washes get bit once every 3-5 minutes with no protection. Seems like a simple solution to me, but they seemed to be hooked on "strawberry extract" soaps. Go figure.
However, the lemon ucalyptis worked "barely passibly" in wooded areas with lots of underbrush - which all seemed to have medium to medium/high misquito populations. ie: Medium means I get 15-20 bites an hour with no protection. With lemon ucalyptis I only get 7-10. If I reapply it every 30 minutes I only get half that, so I think it wears off pretty quickly. Then again, it's very humid in Maryland / Virgina (typcially around 95%) and we are hiking, so the sweat may just wash it off...
The lemon ucalyptis didn't work at all effectively in areas within 20-30 minutes walk from swampy areas near the ocean or bays (ie: Had to re-apply it every 10 minutes, and still got 5-7 bites in the first 10 minutes...) However, without protection I got 2-3 bites per minute.
Now for the bad (worse ?) news. The lemon ucalyptis didn't deter biting deer or horse flies in the slightest. I also concur with the previous poster - knats / no-see-ums LOVE the stuff. They don't bite much, but they are constantly getting stuck in your eyes when you blink, and swallowed if you breathe through your mouth. Without the lemon ucalyptis, the knat problems were only 25-33% as bad.