Depends on what kind of classifieds you're talking about. Craiglist is good because it has so many people subscribed but you have to use your judgement otherwise you'll be banned. Overall, I say go for it because it is cheap and much more effective than classifieds in say local newspaper. Once again though, if you're trying to target local customers you'll have to do it on local websites.
I am a Shaklee Independent Distributor and I would like to market health care products thru classified advertising online.