I don't think they work unless you add your link to a lot. I've been adding my forum to directories for a long time and I've not noticed drastic improvements, but it is getting there.
Directory submission does not have a good traffic and conversion but it helps you build backlinks. I only submitted my site on directories with high PR and don't rely much time on just submitting to all of them.
directory submission not useful for heavy traffic but if u do that do it manually use local directory with high PR
Thanks
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Telecoms Product Supply