I built one from a Dick Smith kit years ago, and it stopped me getting bitten in my bedroom, no idea if they work well outside or not, but this one did in my screen-less house. But who knows if it was co-incidence?
However the research seems to point to currently available models not working at all really:
http://www.straightdope.com/columns/...repellers-work
Quote:
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The U.S. Environmental Protection Agency conducted two years of tests in the mosquito-ridden area around Chesapeake Bay, trying virtually every type of electronic repeller they could lay their hands on. Not a one had any noticeable effect on mosquitoes, or, if it's any comfort, on humans (although some say the latter question hasn't been studied in sufficient depth). Subsequent research at various universities has pretty much borne out the EPA's findings. Both the EPA and the U.S. Postal Service have gone after ultrasonic repeller makers for making unsupported claims for their products.
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What definitely works? N,N-diethyl-3-methylbenzamide (DEET) and permethrin-treated clothing.
http://annals.org/article.aspx?volum...ue=11&page=931