I really like RSS feeds. I have had great success with them on a couple sites. Though I would use something maybe like cinvin. I really enjoy it and it serves as a nice sitemap for spiders and such.
If youd like to see it here is link to it on current site.
Code:
http://arguecrazy.com/cv_rss_feeds.php
It is a nice little addon for vbulletin. And it adds little button down at the bottom. But, I have gotten alot of subscriptions on bigger sites using RSS. It just takes time and have to find the kind of content they are looking for. I think if you use it in a targeted way its much harder. Like a specific section or blog.