Any blow dryers can potentially cause frizz. Especially if you have mixed-race hair, or naturally curly hair. However, some people might benefit from blow dryers if their frizz is a result of uneven drying (hair that is frizzy while in the process of drying, but ends up relatively straight afterwards)

Anais

I think they're meant to do the opposite...because the stream of ions smooths down the cuticles so the hair shafts look shinier.

However, Anais is correct too, the general effect of any heat styling will be to dry out your hair and can therefore result in frizz.  You can minimise that by using moisturising shampoo and conditioner, regular deep-conditioning or leave-in conditioning treatments and a heat protective spray before blow-drying.  Also blot your hair gently with a towel when it's wet and leave blow-drying until it's 80-90% dry.  Hope this helps.

What it's probably doing is getting rid of static... which is cool.

I just thought of something and wanted to add... Gisele, maybe you could find a friend who has an ionic dryer and "test-drive" it before you spend the money for your own.

Anais

I have an ionic hairdryer and to be honest I don't notice any difference to my old one, but I don't blowdry my hair very often and my hair isn't frizzy, so maybe someone with hair that is prone to frizz would notice the benefits of the ions.  I think Anais's suggestion to 'test drive' one is a great idea!