I tgp, free-site, and SE every day and as such it's an additional source of traffic and from time to time sales too...depending on the mood the net is in and if i hold my mouth a ceritan way that day....
so yea it's worth it...and if you do it daily, you can build somewhere in the range of around an extra million surfers a month to you sites in total ..after about 6 months or so of it....so it does build..