Cons: No scientific evidence of effectiveness
In the spirit of full disclosure I will state that I do not own any of these units, however I thought that the below information should be conveyed to anyone interested in purchasing one of these units:
This is a report on the Federal Trade Commission website about these products.
If you go to the Global Instruments, Ltd website (the manufacturer) and read their FAQ:
you'll note that they currently claim that it repels "Rodents only!" This is a direct result of the FTC action.
Just keep that in mind. Spiders, roaches, etc. have not been shown to be affected by it.