For our site, traffic from Google represents less than a tenth of our referrers and is eclipsed even by a DMOZ listing. Splashing out for paid inclusion in Inktomi, FAST and Teoma hasn't resulted in noticeably greater traffic.
Like Tigger says, some of the smaller directories can help in both referrals and link popularity. Submission to DMOZ and Yahoo should certainly help your traffic drive and if you're a non-profit making site, give Zeal a shot - otherwise a paid submission to Looksmart may be worth considering, although I know others may disagree with that one!
Just my two cents worth.
What are the 5 most important UK engines? [webmasterworld.com]
My list of UK search engines [webmasterworld.com]
Half decent submission list (suited to sites that sell something) [webmasterworld.com]